I don't really know exactly how Henry trained but my college roommate, also a WSU Kenyan, trained much like some of the things I've seen written about Henry (running up and down a hill for an hour or two, etc). He never, and I mean never, ran on a track. Did not like to run on pavement. We'd drive a truck out to one of the dirt farm roads in Pullman and start there. I tried training with him because I wanted to learn how to run like the Kenyans but it was so mind crushingly boring I just couldn't do it. I'll never forget the time I did the hill run with him, thinking it was gonna be hill intervals where you run up hard 8 times with easy jog down...nope, just run up like normal run pace (1 minute up) and then down at normal run pace. I thought it would be easy. I was bored for the first 15 minutes (we were going 1 hour). After 30 mins I started getting tired. At 40 minutes I was hoping to make it to the end. At 45 minutes I ran across the street to a pizza place, straight past the hostess to the toilet, and detonated an A-bomb into the toilet because my bowels were spasming uncontrollably. Needless to say I never did hills with him again. He only ran about 359 for 1500m but ran 2:08 high for the marathon when the WR was 2:06:50 or so.
So doing lots of slow miles with some Rono style hills works for some people, it makes you tough as nails.