[Cherno]

Out of curiosity, and because it is an unused feature, I created a fitness boost block on a test map. The block can be chosen in the editor, but there is no way to set the healing value, so I had so manually edit the map file's bytes at the right position. A value of 2560 restores 6 % of Miyriel's fitness, so a value of ~426 equates to one point of restored fitness. I chose Miyriel because she has a Physique rating of 100, which I assume to be equal to the maximum amount of fitness/health.
As for using a fitness boost block: It appears like a forcefield/teleport/exit, and when stepping into it, there is a quick green flash on the character's screen, as if using certain Psi-Amps or possibly teleporting, the teleport sound effect, and the character remains on the cell he or she entered from (or is moved back there). The character's fitness is restored as described above.

Now with that figured out, I can simply add a field in my editor that lets the user set the amount of restored fitness.