Hi Jack – going to talk with regards to exercise. THere’s whole books on thiese kinds of things, but most programs for endurance athletes advocate building sufficient base fitness (by doing lots of steady mileage, usually over the winter) before progressing to faster and harder work, which helps sharpen that fitness to bring you to peak performance for your race season, after which you take a break and then repeat again. Do this for numerous years and you’ll build fitness gradually.
If you want to build fitness more quickly you can also go for intervals – short blocks of rapid, hard activity (eg sprinting) followed by a period of recovery (eg jogging). Repeat said blocks multiple times for 15-30 minutes – as time goes by your hard intervals can be made longer/more numerous.
Don’t do these all too often though since you might end up burning out a bit in the long term. When you think about it sports like football or closed circuit cycling emulate this model a lot – short bursts of activity followed by recovery.
In all cases it’d be best to do some proper reading on the subjects and make up your own mind – when it comes to fitness there’s no easy fix – it’s all about consistent, paced and progressively developing work (just like a PhD!)
I found Brain vs Brawn to be a great introductory read, and probably got me doing intervals, which led to me being able to try out quite a few sports (boxing – although I never boxed someone, then transitioning to running and now cycling, with a focus on endurance+long distance rather than short distance)