But, if some external force compelled the person in question to run several miles every day, lift heavy weights, sleep a lot, and eat perfect food, that person

But, if some external force compelled the person in question to run several miles every day, lift heavy weights, sleep a lot, and eat perfect food, that person would probably be meaningfully less depressed within half a year. Obviously with feedback being what it is it's not that simple, but still. Speaking as someone with clinically diagnosed depression btw. It's a lot of unpleasant effort to kill depression but it's not impossible, and it's the only way to do it. Drugs and talk therapy can help a lot but fundamentally the change has to happen of one's own volition