Ive run many marathons. Ive completed even more 100-mile, or century, rides. Some marathons have felt easy, others have felt like Im treading shark-infested waters with an open wound. Century rides are a bit different. They invariably take longer than running 26.2 miles, bringing with them greater challenges around fueling, nutrition and mechanical mishaps, but, riding 100 miles consistently inflicts far less impact on the joints and tendons. Both are rites of passage within each discipline, but which is harder? Can they actually be compared? Given the impact involved in running, most would probably immediately come down on the side of the marathon; however, it's worth considering the fuelling and pure admin involved in spending around seven hours in the saddle. To help me answer the question, I spoke with strength and endurance coach Ed Norman of Calibre Athlete. For this feature, we focused on the most common forms of each endurance feat: a standard road marathon and a standard undulating road century, both completed at pace which keeps the heart rate around an endurance-focused zone two, with no drafting. DURATIONMarathons place huge stress on joints and ligaments, but 100-miles is just a really long way (Image credit: Future)Looking back in the mists of time, my first century took 7.5 hours. I remember how completely broken I felt: how unresponsive my legs were in the final 20 miles, how my back howled, how my backside grimaced and how my feet begged for mercy.Seven hours is a solid time for a fit cyclist to complete 100 miles, but others could be going considerably longer, which creates a unique endurance challenge, Norman says. Data backs up his sentiment; having crunched the numbers across Strava and Garmin Connect, we know the average cyclist is travelling just under 30 miles (46km) at a time, at an average speed just under 15mph (24k/mh). If that cyclist takes on a century challenge, its fair to expect their pace to slow. The biggest factor with cycling is the sheer time spent on the bike. You're asking the body to maintain an efficient position for hours on end, Norman says. While a marathon may take considerably less time than a century ride the average duration across both genders being 4hrs 30mins Norman argues that the nature of the effort is very different. If you hit a downhill on a bike, you can let the bike do the work, whereas on a run you are absorbing even more impact, even if it's a gradual descent. The energy return you get from concrete or asphalt when youre running is very little.Hardest for duration: 100 miles When it comes to duration, the century ride is the bigger challenge. Even relatively strong cyclists can expect to spend seven or more hours in the saddle, while most marathon runners will finish within three to five hours. The sheer length of time spent riding places unique demands on comfort, endurance and pacing. As Norman puts it, you're asking the body to maintain an efficient position for hours on end.TRAININGRunning involves impact, and requires a gradual build up (Image credit: Future)Preparing for a century ride is largely about accumulating saddle time and adapting the body to spending long periods on the bike. You need to get very specific; specific time in the saddle building up to around two-thirds of your target distance and specific positional durability on the bike, suggests Norman. Indeed, its the position you hold on the bike while generating a smooth cadence that will increase speed and efficiency while mitigating the chances of picking up an injury. Time in the saddle is key. As Norman concludes: You can't really cheat your way to that sort of distance. The build-up to a century varies according to your current ability, but for someone who regularly rides 20-30 miles, a lead-in of around six weeks should be enough to elicit the necessary muscular and cardiovascular adaptations.Running, however, places greater demands on the musculoskeletal system and requires a more gradual build-up. Norman recommends patience. You need to be incredibly specific around your training, Norman warns. You need to have a progressive plan, similar to biking, but the lead-in to a marathon needs to be longer, in my opinion. You need to have at least four to six months of a base to build into marathon training.HARDEST FOR TRAINING: MARATHONWhile a century ride demands more hours of training on the bike, marathon preparation is arguably more complex. The impact forces associated with running require adaptations in the muscles, tendons and bones that simply cannot be rushed.Norman's view is clear: although the marathon is shorter in duration, the exertion rates are much more demanding on the body from a musculoskeletal and bone-adaptation perspective, meaning the training build-up needs to be considerably longer and more carefully managed.NUTRITION You'll have to hone your nutrition strategy to avoid a 'bonk' on the bike (Image credit: Future)If youre an experienced cyclist, youve likely suffered the afflictions of the dreaded bonk. The bonk is a term used when glycogen stored in your muscles is running dry and you hit a wall.A century ride is fertile ground to experience this if you get your nutrition wrong, and theres plenty of scope for error over seven or more hours in the saddle. Norman highlights the importance of taking on carbohydrates over such a protracted period of riding. "A century ride is long enough that your nutrition strategy becomes a major part of the challenge. You need to think about fuelling across a much longer duration and that needs to be practised.In theory, consuming 90g of carbohydrate per hour (the recommended quantity for those riding or running for more than 90 minutes) appears a relatively easy proposition, but as the hours accumulate so too does the risk of gastrointestinal distress and the body being unable to process the fuel youre taking on. This is where many ultra-distance riders come undone; under-eating or taking on calories that your body rejects is one of the most common ways to DNF. Indeed, a 75kg rider looking to ride a moderately brisk undulating century, as many as 6,000 calories could be burned. Marathons, by contrast, although still demanding a good deal of fuelling, are completed over a shorter period of time, usually before flavour fatigue sets in. Flavour fatigue is a very real issue among endurance athletes which occurs when the thought of yet more sweet and sugary carb-loaded fuel makes going hungry more appealing than eating. For marathon runners, where the rule of thumb is 1 kcal per kg per km, this is unlikely to be a problem. The shorter duration will also reduce the chances of GI distress. Hardest for nutrition: 100 milesA century outing, while not quite in the realms of an ultra ride, should still be undertaken with fuel and hydration very much in mind. Unlike a marathon, nutrition for a 100-mile ride should be part of the training plan to make the ride as smooth as possible. Kit and equipmentEven a budget set up requires a bit of investment (Image credit: Future)First and foremost you'll need a bicycle. Then come the cycling shoes, bib shorts, helmet, glasses, lights, pump, inner tubes, tools and navigation. The kit list for a century ride is surprisingly extensive, and will also leave a sizeable dent in your bank account. An entry-level set-up could easily reach and breach 2,000.It's unlikely that your chain breaker or third spare tube will ever leave your saddle bag, but a belt-and-braces approach is sensible when you're potentially several hours from home. The bike itself also requires preparation batteries charged, brakes checked, tyres inspected and bolts torqued.Then there's navigation. Unless you're following a well-signposted event, a century ride often requires a GPS head unit and route planning. Just don't forget to charge it.First and foremost you will need a pair of trainers and, well, not a lot else really. Although depending on how deep your pockets are, a good pair of running shoes can be bought for as much as 150, with entry-level models sitting somewhere around the 50 mark. Obviously a sweat wicking running shirt and comfortable pair of shorts are non-negitiable. Some marathon runners go that extra mile and splash out on a pair of compression socks. And then of course theres the all-important watch because if its not on Strava, as the saying goes, it did not happen. Hardest for kit and equipment: 100 milesCentury rides require forethought, lots of planning and pockets bulging with spares and tools. Theres also the added complication of having to transport your bike to the start of the ride or race and then clean any crud picked up over the course of the outing. Running? Just put on your shirt, shoes and shorts and step into the fray. Simples.The final scores on the doors...The bike ride edges it; no doubt runners will disagree (Image credit: Future)From a logistical standpoint, there is only one winner. The century ride is rife with "what ifs" mechanicals, navigation errors, fuelling issues and weather changes. The marathon is, at least on paper, beautifully simple. Just run.It is when we introduce impact and training load that the waters become murkier.Training for a marathon is anything but straightforward. Without a structured build-up, there is a genuine risk of injury. I've made the mistake before, rushing into longer runs and limping away with shin splints or hip problems. As Norman points out, marathon preparation needs patience, progression and months of conditioning before race day arrives.Cycling presents its own unique challenges. The glutes, quads and core must be conditioned, and perhaps more importantly, riders must learn to remain comfortable and efficient on the bike for seven or more hours. That asks questions of the entire body, not just the legs.So, is a marathon the running equivalent of a century ride? In some respects, yes. Both demand commitment and consistency. But they are challenging in different ways.Objectively, the marathon probably edges it. The training build-up is longer and under-preparation can come with severe consequences. Norman's verdict is clear: despite being shorter in duration, the marathon places greater demands on the musculoskeletal system and requires more careful preparation.Subjectively? I'm still not convinced. As I crunch my way up another 12 percent climb on a ride that was meant to be an easy lunchtime spin, I'm reminded that seven hours on a bike can feel like a very long time indeed. For me, that's enough to give the nod to the century ride.