Han, I have looked for the Ingebritsen training quite a bit and cannot find reliable information. Would love a point in the right direction.
A lot of the slower than expected running sounds quite similar to what Marcus O'Sullivan has done for years and years at Villanova with his HR and Lactate based training. You would be shocked to see how "gentle" a lot of the work Nova does is, but people still run extremely, extremely well off of it. Lots of running at paces slower than MP for long stretches (what he calls zone running) and LT and track work always under control using a combination of lactate testing and HR.
Marcus is not doing double threshold days and his volume is not excessive, but it's pretty amazing to see the results he gets with the training he uses.