I did about four and a half again today. I should be ready to start ramping up the intensity next week...

Indy - Using the HRM is a good idea, but I also had the thought that rather than trying to lengthen your run intervals, you might try shortening your rest intervals instead. Intervals of 3-5 minutes are some of the most difficult you can do - I've even read a few things that say just not to do them, because (according to those coaches) the amount of muscle damage you do in long intervals isn't worth the gain.

Maybe try a good thorough warm-up and then a series of one-minute sprints with one minute rest. Start with just a few, maybe five, see how you feel. Jog a bit afterwards or just walk it out.

I forget, do you practice Chi Running? What's happening in your body when you find yourself over-exerting? Quickening your cadence unintentionally, leaning farther forward than you meant to, overstriding? Have you done much work on the Chi Running concept of "gears?"