+1 on Chi Running.

And something that's more implicit than explicit in Chi Running - foot strength. Even if you don't decide to transition to barefoot running, you need to have a good foundation, or all the rest of your body mechanics will be off. Yoga and other barefoot regimens - and just walking around barefoot or in Vibram Five Fingers shoes - are good for building foot strength. That's assuming you have healthy bone structure in your feet (I still haven't found an expert I trust to tell me about mine - but that's another thread).

Having come in two years from running 3-4 miles only when I traveled, to running 3-5 miles once a week, to my first half-marathon yesterday, IMO you need to do any activity at least three days a week to be able to gain fitness and learn good form. I don't necessarily ride three and run three every single week - it depends on what I'm training for at the time.

The general rule is increase mileage by no more than 10% per week - less than that if you're having trouble with injuries. And be diligent about stretching immediately after a run. A thorough stretching routine takes me at least 20 minutes, sometimes more, which is kind of annoying if I'm running short, but I think it really helps ward off injuries.

Good luck and have fun!