Here is a great example of a training session for XC and Endurance racing. This workout is using power at zone 7 (150% ftp) for 30 sec on, and 30 secs. off. There are 4 sets of 5 intervals each with a 5 min rest between interval. Each set includes 5 intervals at 30sec power zone 7, then 30sec power zone 1 with each spinning.

Why do these type of intervals?
- Improves endurance
- Improves Peak Power Output
- Improves VO2max
- Faster recovery time between training sessions
These are best done during the race/peak phase of a training plan. I prescribe these once a week for three weeks leading up to a B or A race for some of my clients.