After a solid marathon, I crossed the finish line, said never again, and then three weeks into recovery, I was googling 50Ks and even 100Ks. So here’s what I’ve pieced together about what’s actually different between a marathon and an ultra and what seems worth knowing before you sign up. I’m an amateur figuring this out as I go, so take all of this as one runner sharing notes, not gospel and certainly not medical advice.

It sounds simple, but it is essential to understand that a marathon and an ultra are not the same event at a longer distance. They’re physiologically different enough that training for one can apparently work against the other. A marathon is sustained hard aerobic effort, 70 to 85% of your VO2 max, and learning to carefully manage glycogen store for 3 to 5 hours. Everything is about holding pace under that specific kind of stress.

An ultra is something else. The pace drops, the duration stretches, the terrain potentially gets technical, and the physiological stress accumulates in ways that have less to do with cardiovascular intensity and more to do with time, mechanical load, and the slow breakdown of systems you didn’t know were involved. The pain and stiffness in my muscles that I felt after my first ultra, for which I prepared just the same as for my marathon schedule, was unreal. I was totally underprepared. Just completely different animals. As a fun fact, research shows that peak performance age in ultras is 35 to 45, compared to the late 20s for marathons. Different sport. Different everything.

The body damage is also on a different scale. Creatine kinase can reach values of 100,000 to 200,000 units per liter after an ultra, depending on fitness level and race distance. To understand if that is a lot or not, let me just say that the normal reference range tops out at around 200 🙂 Your kidneys have to filter all of that out, which is why an ultra marathon often leads to a temporary reduction in renal function, and why cola-colored urine post-race is a signal to take seriously. Gastrointestinal bleeding after an ultra is not uncommon, and liver enzymes can increase considerably during the race. Most of these changes are temporary and normalize after the race, but they’re worth knowing about before you go in blind.

The gut is its own challenge. The most successful ultra athletes don’t rely solely on sports nutrition products; they carry real food with mixed macronutrient profiles to support sustained appetite. Sandwiches, boiled potatoes with salt, rice balls. Things with texture. The stomach needs training as much as the legs.

Training shifts significantly too. Pace stops being the primary variable, time on feet is now key. A five-hour run on technical trail that only covers 35km is a good ultra training day. Back-to-back long runs on consecutive days are essential, training your body to move on already-fatigued legs, which is what the second half of any meaningful ultra actually is.

For what it’s worth, a 50K with a solid marathon base is genuinely approachable for most people. Slow down in training, more than feels right. Practice eating on your long runs and find out what your stomach actually tolerates at hour four. Accept walking the uphills without treating it as failure. And go into your first ultra curious rather than performance-focused. The time goals can come later, once you know what you’re actually dealing with. Now, for that 100K, go in well prepared, it is not just a longer marathon. Not for your body and neither for your mind. You have to take that training seriously and treat it as its own discipline, not a longer marathon.

Before we go: What works for me might not work for you, and I’m very much still figuring this out. But that much seems to hold across most people who’ve done it. I am not a doctor, do not take this as medical advice etc.