I started running ultras at thirty-two for unflattering reasons. I wanted to feel like I was good at something physical, and I had the time. I have since finished four hundred-kilometer races, several shorter ones, and dropped from two. The most interesting thing I learned has nothing to do with running.
Ultrarunning is, fundamentally, an exercise in managing your own mind for a very long time. It turns out the techniques transfer.
The pacing problem
In a hundred-kilometer race, the first thirty kilometers are easy and feel like a betrayal. You're full of energy, you're surrounded by other runners going too fast, and the most disciplined thing you can do is hold back. Almost everyone who blows up later did so by running the first third too hard.
Deep work has the same shape. The first hour of a serious writing session feels too easy. You can produce a lot. The temptation is to keep producing. The discipline is to stop, because what you've written needs to be read back in a quiet room before the next thousand words go on top of it.