Fitness Skill is a free mod for the game Valheim. It adds a skill called Fitness, which will increase the maximum amount of stamina and regen you can use. It is increased by using stamina.

This is simple and easy to install, but first, you must have the Skill Injector mod and BepInEx Valheim.

The default config file was created by the developer, and it is available online for everyone to use. It is also an open-source code, you can use open it on GitHub and make modifications as long as you credit the original creator.