Finding a balance at fifty+

At fify-two am increasingly aware of my inability to maintain the level of activity I am used to. It wasn't long ago that I could play two competitive squash matches in a day and repeat that two or three times each week, cross country mountain bike in tough terrain for 70-100 km each weekend and train in the gym up to three times each week with no real tiredness. Now, if I go to the gym for a workout, play a decent squash match or go for a decemt ride I find I am often exhasted the next day and sometimes for two days afterwards and more allarmingly I am picking up all manner of minor ailments so i guess my immune system is suffering too. I have been playing with diet, intensity of workout and supplements (thinking that things like magnesium might be lacking) but I can't hit on a balance that allows me to do what I want. Any tips?