Setback?

I think the cramps I experienced during my 20 miler last week were something else. The lower hamstring area of my left leg still hurts a lot when I try to run on it. The good news is that it doesn't hurt when I walk on it, and I'm able to use the elliptical trainer and bike at the gym with no problems. Right now I'm trying to figure out if its a slight muscle pull or maybe an incipient stress fracture. When I had a stress fracture of the femoral neck (I know, a totally unusual place for a stress fracture) several years ago, it felt like what I imagined a groin pull would feel like. Of course I tried to run through the pain until I couldn't walk any more and ended up on crutches for eight weeks. That was one long summer, but by the end I knew which motorized carts at the grocery store were the fastest! I'm going to continue to sort of take things easy by getting my cardio at the gym, alternating the bike and eliptical. They aren't as effective as running; I just can't get my heart rate up on the bike, and although I can get a good heart rate on the elliptical, my feet fall asleep after about 30 minutes. Perhaps I need to participate in a couple of spin classes, having someone yell at me usually gets me working harder.