HI. I Am A Pretty Healthy 57 Yr Old Male.

HI. I am a pretty healthy 57 yr old male. I was running Sunday night and all of sudden the calf muscle in my left leg just seized up. I live on a hill and could barely make it back up to the house. By Monday night my entire lower leg was swollen. Immediate thought was a blood clot. Calf muscle was hard. I checked my pulse in my ankle and there was a pulse. No numbness or tingling. No shortness of breath. Elevated and Iced. Next day went on a business trip to Philadelphia. Painful to walk. Cannot take a full stride. Tried to stretch it out on a treadmill but I could not even stay on it 2 minutes. Is this just a severe muscle strain or worse?? I exercise an hour almost every day. I can not do anything aerobic now. Please advise