If you're not used to running, it takes a LONG time to build up mileage without injury. Cycling isn't as hard on the body... Mileage increase isn't comparable to running. With cycling if you get to 70% of a century distance once, you can do the full distance. I don't know that I would do that running.

Granted, my long run is only 3 or so miles. But I've been working on that a long time. I've been spending a few months just running 2 and 3 miles before I try to proceed foward to 5 and 6.

Over at coolrunning.com people keep saying to only increase your distance each week by 10 percent. Then every couple weeks scale back the mileage again for a recovery week. It's a pretty good site for running info, just like this website is good for information primarily of a cycling nature.

Guage your body just like with cycling. It just takes less running to equal the same impact as cycling. If your joints are hurting the next day, rest them longer or scale back the effort and do a recovery day of walking or something.