Nice job! I have some skimo friends that are doing the following treadmill routine.
- 10 min warmup
- set your treadmill to max incline
- 30 minutes of 60 seconds "on" (10:00 pace) followed by 60 seconds "off" (20:00 pace)
- make your treadmill flat again and cool down
I have yet to try it but 15 intervals sounds hard!