Hey trekkie820.
My advice to you (from someone who races both running and cycling) is to build up to more than 3 miles per day. Running is great for cardio, because unlike being on the bike, there's no way to rest. Even on downhills, there's no way to "coast" while you run. So for maximum cardio benefit, try running a little bit longer each week to build up that high-intensity cardio, over a longer period of time.
Also, running tends to have a slower leg cadence with a lower range of motion than cycling. To compensate, try throwing in a little interval training. It doesn't take much, maybe one day a week of sprinting for 100 yards, then jogging for 300 yards, 4 or 5 times. Concentrate on developing fast leg turnover.
Also, to keep your "biking" leg muscles strong, add a day a week of hilly terrain. This forces you to use more quads, less hamstrings, thereby strengthening more of the muscles you'd use for riding.
There's no substitute for saddle time, but if you plan your running workouts, you'll be fine.
