[xiaoma]

Nearly every high school cross-country runner goes from zero to five days a week of running instantly. I built up to over 62 miles and six days of training a week in the first season and I know plenty of others who have done the same. Not only that, but I did the same thing again 15 years later and 60 pounds heavier.

The human body really is designed to run. Most injuries are due to pushing to go too fast. If you keep your heart rate at about 60-70% of max, you'd be surprised how quickly you can ramp up mileage without hurting yourself. The key is to never push yourself so hard that you feel like you couldn't do a workout the next day.

Cycling works almost as well from a cardio-vascular standpoint, too. It's not weight-bearing, though, so it won't help improve your bone density or strengthen your joints like running will. You could probably compensate by doing a bit of weight training. In the end, it comes down to getting off your butt and moving.