An interval session is a fantastic way to increase your fitness. Start off by warming up
to your comfortable pace (CP). Once there increase the pace between 15% - 30% (if you are
using a heartrate monitor refer to the article Heart Rate) for bewteen 1 - 4 minutes, then
decrease to your CP for between 1 - 4 minutes and repeat both.

Here is an eg:

CP 4 mins
+15% 3 mins
CP 3 mins
+30% 2 mins

Repeat 3 times = 36 mins

You can then calculate how much of the interval you use to increase your fitness.

So +15% for 9 mins and +30% for 6 mins = 15 mins out of the 36 mins were spent in
an increased fitness zone.