This is all some interesting info. Right now I'm just going to continue my current regime of 45-60 min. doing spinning videos with my HR in the 120 -130 range for 5-6 days a week. Yesterday I did about 30 min. of a (50 min) video and then tried Aaron's 30 sec. sprint method. I ended up pretty much at the 220-age formula. Hard to believe that's anywhere near accurate since I'm generally working in the 120-130 range and that's at 80-87% of max. Shouldn't I be feeling seriously stressed at that %? FWIW at that % I can easily have a conversation and don't have any particular lactic acid burn. There's so much about this I don't know. The good news is the prospect of starting a training program has got me training consistently. It's kind of like what Road Fan said.