Looking for a fitness bike since I am wearing down the lower back from running. I'll be alternating running and biking. I probably won't bike for more than an hour, except on rare occasion. I'm about 200 pounds and 5' 10". Looking to spend $600 or less, if possible. Any help on models and where to shop (online or otherwise) would be most appreciated. I live in Kensington, MD 20895.