i do mine on a stationary bike with 30 seconds of work at a time - i just don't have 60 seconds in me yet! mine looked like this when i started:
120s warm-up
30s work
60s rest
30s work
60s rest
30s work
60s rest
120s cool-down
i have since added two more work sessions. i'm certainly no expert, but it seems to me that adding work sessions rather than lengthening my current work sessions is the way to go, at least for now. i have also boosted the level at which i am cycling.
karen