My guess about how healing magic is used here was correct, but it really does not make much sense.
Essentially, this is how Tsunade's and Sakura's Byakugō no In worked in Naruto. You know, the rhombus mark on heir foreheads. Or rather part of it that helped Tsunade maintain her youthful appearance. Since the technique was added to Naruto during times, when techniques had rules and limitations, its core was that user stored chakra in a seal in a continuous manner. This required great chakra control and precision in using it (which is also requirement for medical ninjas in the series). This was, essentially the limitation of the technique: you need to be precise and you need to have enough chakra to be able to store at least some portion of it into this seal.
This show is clearly not Naruto, and not even Boruto (which still has at least some limitations to their new techniques), though. Healing magic is used here to strengthen one's own body, essentially accelerating gaining physical strength and stamina and all that. On the surface it makes sense: physical activities do damage muscle fibers, and muscle growth is achieved by regeneration of those fibers and creation of new ones. Since healing accelerates regeneration of the body - this is a perfect match.
Problem is that in this show there does not seem to be any limitations asides from you needing to poses affinity to healing magic. Usato was not taught any kind of control of the magic (although maybe he will be in the future), let alone any finesse with it. He also is using the magic without any costs, making him able to run and do pushups practically indefinitely, as long as his current muscle mass allows him, I guess. They are, essentially, forgetting that the magic still comes from within him (which was confirmed pretty early in the episode), meaning, that there is a limit, which would probably be dependent on how much energy he has, and that deplete with physical activities.
Yeah, I know games have taught us that stamina and mana are separate, but unless mana is stored in a separate container (or organ), which is somehow connected "to the world" and is entirely independent of the body - mana would be dependent on your stamina. Or more like on how much you eat, how much energy you exert passively, and how efficiently you exert that energy when doing stuff.
As result this show's system makes sense on one hand (the physical training being aimed at increasing stamina), but it fails on the other hand, when they both should be connected. In addition, I am not sure the "Grand Blue" kind of dynamics and humor work here. I think the story, the world, even the magic system would work way better with a more serious tone, maybe with somethin like what "Shield Hero" had in its 1st season.
