[whichfawkes]

For me, the main benefit of exercise is immediate. However - the magnitude of the immediate positive effect is somewhat related to the intensity/duration of the exercise that I can tolerate.

So I would say something like "A 5k run would make anybody above a certain fitness level feel pretty good". Like, in a way that you'd notice immediately during, and after. For me, there's runners' high and good body-chemicals and a sharp mind the rest of the day. There's not as much of the good stuff if it's like a real strain to run that far. There's an optimal level of physical stress. Taking a sedentary person and chasing them through a 5k run with an electric cattle prod them is not going to help them feel good. Similarly, someone who regularly runs marathons is not going to notice much of an effect from a short walk at a slow pace.

How to balance this out (how much exercise do you need to feel good) is a very individual thing. Practically speaking, you just have to try stuff and get to know your body.

I will say though that marked changes in physical condition can occur over more like weeks than decades. So if you go for a walk and think "well that was a little bit nice, but not worth taking an hour to do", maybe invest the time to build up to something more intense like cycling or jogging, because, as anyone who exercises intentionally will tell you, it seems to be worth it once you find out what's right for you.