I've enjoyed reading a lot of the training posts here, I think I'm gathering that the 20 minute interval can be a great training tool, useful for improving the 'functional threshold power'. I typically ride a short loop in a local derelict city park, I have cranked out a sub 20minute lap, but I think I'm 'between peaks' at the moment, and a typical fast lap time is 21-22 minutes. The loop has 4 short hard hills, 1-2 minutes each, and a couple of shorter climbs (about 400' of total climbing), with lots of twisty narrow sort of rough trails. I have hit 190bpm running up (cx training), but my usual max is about 180bpm, I can keep my heart rate above 160 the whole loop if I'm hammering (probably between 162 and 178). I'm wondering what you more experienced guys think about doing laps as intervals?; one hard 22 minute lap, then an easy 28min lap, then another hard lap? The hills are hard enough on the easy lap that it's difficult to keep the heart rate very low. Is that range (160-180) close enough to call it an interval?, or are there too many downhill 'rests' in a loop like that? Would I be better off doing a hard lap, leaving the park for 10-20 min for the easy part, then returning to the park for a hard lap? How long between hard laps?, I could get 3 hard laps in if I did 10 min easy in between... I just upped from a podium regular cat2 to a low cat1, and my schedule allows for about 6hrs per week. -thanks for the input!