I was chatting with my new "trainer" and we were discussing the cardio part of my workout. I told him that I run two to two and a half miles a day and that was it as far as cardio. I run outdoors so I'm able to change courses and incorporate hills and such. Anyway, he suggested that I run for time and not distance. I was finishing my two miles in about 16 minutes and I was informed that I wasn't going to make much progress since I wasn't keeping my heart rate up long enough.

I don't mind running for time, but keeping that up for 30-40 minutes.. I could run to the next town haha. When I get home I'm too exhausted to do my pushups/situps/stretching. I've tried walking part of the course, but then I feel my heart rate drop too much and have to work harder to get it up again.

The question I have would be, what can I do to maintain my heart rate for the duration of my 40 minute cardio session without feeling exhausted at the end? Any suggestions for variations of walking/jogging/running/sprinting would be helpful.

Thanks!!