Fitness is a skill in Survivalist. It determines how much you can carry and your fatigue levels. The higher your fitness skill is, the more you can carry and the longer you can sprint without getting fatigued.
Increasing this skill
There are a couple of ways of increasing the fitness skill. You can sprint for long periods of time or you can carry around Health and Fitness Guide that increases Fitness by 1 for each guide.
According to the developer, any action that causes fatigue increases the fitness skill as well as carrying a lot of weight while walking. Having characters haul items automatically will slowly increase their fitness.