Originally Posted by Brian Ratliff

You should be blown after two. Warmdown and go home. 175bpm is probably a good starting place. It really depends on the individual though. Mine is 170-180. But I've got a teammate who would do his at 180-185, and another who's at 150-160. It really varies by person and age. If you are young (your max suggests you are), you might try 180 to start and adjust from there. The objective is to do the entire 40 minutes of intervals at the same HR. If, at the end of the set you have more left in the tank (you could have continued after the second interval), do the next set (on a different day) at a couple bpm higher HR.

Again, it'll take a couple weeks to get into rhythm. Don't be afraid you are somehow "missing" training; no such thing. Any intensity leads to improvements in fitness. The worse thing you can do is panic and think you aren't doing enough and to compensate by trying to cram in too many miles in a week. You'll fall into the fabled 80% trap where you are doing an awful lot of riding every week, but everything is being done at 80% effort. You never train the top end, which is what is required for racing.

These intervals should be at 100%, and then you go home; even if you feel you could have ridden longer afterwards. The intent is to train your body to go harder than it's gone in the past. To put more force to the pedals for longer periods of time. It's not the intent that you are pedaling squares at the end of the workout or to go so hard for so long that you can't walk the next day. You probably won't even feel it the next day because you are training primarily your lungs and heart, not your muscles.

Hopefully this is putting things in perspective. The training programs you read about in the books are mostly directed at riders who've been training and racing for several years and are trying to further hone an already sharp knife. In fact, in Friel's book (The Cyclist's Training Bible), he even says that if you are in your first two years of training, to ignore almost everything he's written and keep it very simple.