I wouldn't worry too much about distance. Get out there and get your heartrate up -- if you're riding on a trail without traffic control, which it sounds like you are, try to keep a steady pace and you'll burn calories like mad and your heart will get really healthy. Don't concern yourself with speed so much as a steady pace. You will just nautrally start riding farther and faster. And if you really want to build leg muscle, sprint up hills.

These two strategies are pretty much what I do. My commute is where I get my sprinting in and our weekend and weeknight rides are long, and steadily paced. At 34 years old, my blood pressure is finally back to normal -- down from 175/115 seven years ago --, my resting heartrate is down by about 20-30 bpm, I can eat (and drink!) whatever I want, I feel better than I ever have in my life, and at my annual physical in February my doctor said "whatever you're doing, keep doing it".