You are screwed I am 63 when I turned 40 that was the first time I really noticed a difference when I was running I always used running as one of my primary ways to keep in shape no more my legs do not co-operate knees and all kinds of little things happen pain injuries etc sucks I have the desire I do not have the body and I am 6 ft 200 pounds
I can tell you that while this sucks, there is a reason why and that reason is that despite the cardio high and benefits you get from running -- it is not good for you time to do as others in this thread indicate
Cycling is excellent swimming is the best and Yoga lots of it I swear by yoga
Here is the best advice I can give you - if you have a knee or muscle issue and think you may need surgery to clean something up = try yoga for 2 weeks doing it every day I cannot believe what it has done for me I had knee clicking, no more after yoga
and diet - enjoy life but if you are serious about making some changes you need to incorporate more plant and veggie foods if it does not grow, do not eat it hard, to do, but even if you do some of this, you will see benefits
I also take glucosomine chondroitin for my joints I think it helps but cannot say for sure