A lot of this is psychological. I was riding on my commute the other day and this really nice looking triathlete type passed me so I figured I would hang with her for a while just for fun. Managed to keep up with her for 20 minutes until our paths diverged and I didn't really feel over-taxed either. When I uploaded the data from the ride my heart was between 170 and 175 for that 20 minutes which really surprised me since the 220 - age gives me a max of 165. I'm not one of these people who ride for the exercise or train or anything like that so I don't think I could do that again on purpose but the testosterone auto pilot seems to do the trick. Now if I only has these women pass me often enough I would get my interval training and could start racing! ...