I'm new to running (started Dec. 7th) and have been trying to integrate some intervals into my running. I have two goals in mind. The first is to lose weight, (lost 50 lbs. so far) the second to increase strength and endurance. In terms of intensity, my HR peaks at 90-95% of MHR (using the kervohen method to determine MHR), but doesn't stay there very long as the intense part of the run is lasting only 20 seconds. During the recovery stage though, I'm still running pretty hard with my HR settling between 78%-85% of MHR. The entire run is 4.5 miles/6 days per week.
My question is should I be doing a couple of those runs at a slower pace for a longer distance to increase my endurance? I find that after the above, I'm always pretty well spent, and while I'm losing weight, I don't feel like my endurance is increasing as much as I would like it to. Any suggestions?