@jespah,
That's interesting, Jespah. I'm kind of stumbling into interval training myself these days. The stumble began two weeks ago, when I realized I was getting close to the point where I could sustain a 5 mph pace. I decided to try sustaining it for an hour, but it didn't quite work. One day I could do 20 minutes, one day 43 , one day 35 -- but never a full hour. That was frustrating.
Then I decided to break the workout into 12 intervals of 5 minutes. I would run four minutes at 5mph pace, which got me acquainted to the rhythm. then one minute at 2.5 mph to recover. Then back to 5 mph. It's much more motivating this way: I'm getting the feel for the 5mph pace, I'm getting to run 48 out of 60 minutes at it, and I see a clear path to my goal: Gradually extend the intervals by taking out the low-speed stretches, until I sustain the speed for the whole hour.
I used to be an LSD man (as in "long, slow distance" of course), but now that I try interval training, it's working very well for me. Highly recommended!