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If you've run a marathon, you can run a 50k. The running is the part you already know. Everything around the running is what catches you.

I've run two. The first was on the Chicago lakefront path in 2012, as flat as a course gets, in 3:58:56. The second was the Grindstone 50k in Virginia: 32.5 miles, 5,020 feet of climbing, 6:29:20. Fourteen years apart, but age isn't what added two and a half hours. The trail did.

If you just signed up for your first one, here's what I'd tell you.

If you've finished a marathon, the extra five miles is not the hard part. The end of a marathon feels like you couldn't go one more step. In a 50k that point doesn't show up in the same place, because you know from the start that you're going further. You pace for it, and you get through it. This is the easy part.

Running is only half of a trail 50k. The other half is rocks, roots, branches, grade, and footing you have to read every step. Find out what your course actually looks like, and train for that.

I didn't. Every long run I did this summer was on flat Ohio road. The longest, 22 miles, gained less than 200 feet. My one real week of hills came on a trip to San Diego in July. A few weeks before the race, I wrote about how little hill training I'd done. I trained through summer heat and humidity too. On race day they mattered a lot less than elevation and terrain.

If you can't get to terrain like your race, find the closest thing: parking garages, highway overpasses, anywhere with actual grade. And get strong enough for it, which brings me to the part that broke me.

I spent the whole build worried about the climbs. It was the descents that wrecked me. Coming off the big ridge my quads went, and after that I walked every gradual uphill because I didn't have the legs left to run them. Rocky downhill was something I thought I could muscle through. I couldn't.

I did some leg work during the build, but none of it was aimed at the quads or at what running downhill demands, and a treadmill can't simulate a descent. Here's the strength work I actually did. For a race that drops, I'd build it around the downhill. The race recap has the full story of how that played out.

A road marathon is predictable. You know roughly how long you'll be out there, and you can get by on a pocket full of gels. A trail 50k is harder to predict, and you'll be out long enough to get sick of them. I was done with gels by mile 28 and finished on aid station watermelon dipped in salt, peanut butter filled pretzels, and a PB&J.

In Training Essentials for Ultrarunning, Jason Koop names GI distress as the number one reason people DNF. Train your stomach the way you train your legs. Eat real food on long runs, find out what sits well, and plan for the point where gels stop working. More on what I carried in What I Carried vs What I Used.

I treated every long run in the build like race morning. Got up and ate exactly what I planned to eat before the race: coffee, a bagel with jelly, a banana. By the time the real morning came, nothing about it was new.

Use those runs to practice carrying, too. Figure out what you actually need on the trail and what just rides along. I'd practice this more than I did. My race setup was one bottle of Tailwind, one bottle of water, and a salt capsule every hour. The tradeoffs between carrying options are in Handheld, Belt, or Vest, and the fueling math is in What's Actually in My Vest.

The ultra cliché is that it's not if you hit a rough patch, it's when. It's a cliché because it's true. Go in knowing what you'll do when it shows up.

I had a handful of tools: positive self-talk, music, and finding someone moving well and staying with them. On the big climb I fell in behind another runner and copied his method. Run the flats, run the gentle downs, walk everything that goes up. On the descent I let an older runner pull me down. I also made a point of staying pleasant. I hit walls and still talked to people on the trail. The strategies I leaned on came out of Koop's book, and I covered them in The Mental Side of Running Is a Skill, Not a Trait.

That training is a big part of why the race went well. I trained my brain for the hard stretches, and it held. I didn't train my quads, and they didn't.

My build was about 20 weeks, on a plan I put together myself from Koop's book. I'd always been handed a plan before, so I wasn't sure this one would do what I needed. Outside of the hills, it did. I wouldn't skip any of it.

If you have kids and a job, you will miss runs. I lost most of a week to my son's baseball practice. It's not the end of the world. Most of what you get out of this happens in the training, not on race day. Missing a few runs for your kids doesn't change that.

After the race, a lot of people asked about my time. It's the wrong question for a trail 50k. Even at my most miserable, there was no point where I wished I wasn't out there.

If you've been running road marathons, this is the race where you get to stop watching the clock. Enjoy being on the trail, and enjoy being on your feet for that long.

Most of this list you've done before. If you've trained for a marathon, you already know how to build mileage, fuel a long effort, and get through a bad stretch. The terrain is the part that's new. Find out what your course looks like, train for it as best you can, and then go enjoy it.

Have a plan, even if you know it's going to get blown up. Mine did. I planned for the climbs, and the descents were what got me. Make one anyway.

And enjoy the training, not just the result. The finish time is one number on one day. The months of early mornings that got you there are most of the experience.