I would say, and I am just thinking about you, that your age may have something to do with it, it seems the older you get the harder it is on your body, I was just reading something in my Triathlon Training Bible about that, the author surmised that early riding would be fine but watch your heart rate closely, and advised not riding after the first 4 or so months.

You can set your bike on a trainer and go all day tho! good luck