30 / 30 is a good workout. You could also do a Tabata interval with 20 second sprints and 10 seconds of rest for 8 cycles (4 minutes).
There is a ski club in my area and I like to do my sprints up one of the hills.
Id recommend the Tabata intervals that he mentioned. We actually did those today but with squats, push-ups, pull ups, and an ab workout.
One sprint workout my academy does is 10 sprints down and back across the gym 20 seconds rest, 10 down and back, 20 seconds rest, usually 4-8 times.