Originally Posted by caloso
Then you might benefit from a long, mellow ride. Some call them "aerobic" or "LSD." Get out and spin a high cadence over a long road. Don't look at you speedometer, just get those legs spinning and don't allow yourself to get out of breath.
+1! Remove the cyclometer and HRM and just roll. If you push your muscles/tendons at all, you're failing. Don't do it.
I'm a bit cautious at my age (38) and would see a physician. It sounds like you're getting a lot of strong negative feedback from your tendons/muscles and I'd be doubly sure it wasn't heart related.
The only time this kind of thing happened to me was as a teenage marathon runner. I would start running and then suddenly fall down after my legs just collapsed under me. My doctor pointed out that I had numerous stress fractures in both lower legs and told me not to run at all and prescribed ibuprofen to manage the healing process. Ibuprofen is an effective anti-inflammatory.
Four weeks of total rest and moderately large doses of Motrin (ibuprofen) had me back up running.