I have chronic fatigue and while cardio can cause a lot if problems, weight training has been beneficial. And if you look at my routine you would laugh as it is about half the time other people spend in the gym. I am there twice a week for 50 minutes and most is rest time. Often there is an exercise you can do a bit more of, unless you are very sick. It is more linear than people think: you don’t need to choose from the binary: sedate/marathon runner pair.