Hey Coach Jeff!
That's very interesting. In my experience, I'm pretty sure I got the intensity down correctly (not exceeding LT) but the total amount of the faster running plus the other increase in mileage was what got me. I believe Smoove made a reference in this thread to not increasing more than one variable at a time and I absolutely did so.
I think the difference between you and me is that you are starting with a better base than I did. I just wasn't prepared to run that long with any kind of intensity at all, even if I didn't exceed a threshold level.
Personally, I'm fascinated to see how you progress following this approach. I think it has the potential to be even more than you are hoping for especially considering how comfortable you are so far.
I really can't wait to see the 2400m test results compared to 6 weeks ago. I had a terrible time running the 2400m test. I kept overshooting and then undershooting the HR target which resulted in a very uneven effort. That totally negated the purpose of the test. Hadd said it should be a very even effort and for whatever reason I couldn't nail that down when chasing an HR target. What I ended up doing was running at set paces instead for 10 minutes each and measuring the HR. That was incredibly easy to do on the track. I was even able to plot my HR vs pace and estimate my lactate threshold HR by finding the point in the graph where the HR stopped being linear. At the time it was around 173 bpm if I recall which was about 88% of my estimated MaxHR. Very fun stuff!
Keep it up! Rooting for you!