Middle-aged men waiting for motivation to start an exercise program have some good news - the exercise does not need to be that vigorous or take that long to do a lot of good. And, if you weren't an exercise dynamo in your 20s or 30s, you haven't lost ground in preventing heart disease. The key is to start now. Twenty minutes a day is enough to get solid benefits -- a 21% reduced risk of heart failure! Inactivity raises your risk 69% for early heart failure. You don't have to overdo it. The…