Surgtech, I DID do this. I'm 55. Never ran before but have been riding 2000-3000 miles for the last five years, and also swimming for the last year. I got bored walking the run in a sprint tri, so took a "Learn to Run" class this past fall with a certified Chi Running instructor. I was 60 pounds overweight at the time (20 pounds down since that start in October). We ran a couple of minutes, walked a couple, for three miles, changing the mix as the eight weeks went along. You really are not coming fresh off the couch, since you already ride, but the couch to 5K program is not a bad way to start anyway. There's a guy who did an 8-week couch to 5K podcast that you can just download into your iPod and DO. Very helpful.
I'm running 8-12 miles a week with no injuries.
First thing to do before all else: get a real fit (doesn't this sound familiar) for the proper running shoe for your feet. Consider consulting a sports podiatrist or PT about running orthotics.
"My predominant feeling is one of gratitude. I have loved and been loved;I have been given much and I have given something in return...Above all, I have been a sentient being, a thinking animal, on this beautiful planet, and that in itself has been an enormous privilege and an adventure." O. Sacks