I try to run 5 miles a day, 5 days a week. This hits the sweet spot of being invigorating rather than over-taxing. Every time I try to do more miles than this, either I injure myself or lose interest.
I've tried weights, aerobics, and yoga, and swimming. But for some reason running is the only thing that gives me the endorphin rush I need.
Wow, great job! Keep that up! Not hard on the knees?
So far so good, knock on wood. But I have injured my hip flexor a couple of times, and it always sets me back for months. That's why I'll probably never train for a marathon. That's all right, though.