I'm sure I'm not alone in thinking that this is fascinating stuff, Lydiard Follower, and I truly appreciate your taking the time to give us your insights.
Having just finished a 12-week period of fairly high-mileage easy running, I've decided to give the hill-springing/leg speed phase a try for about 5-6 weeks. Tried my first hill-springing/downhill striding/windsprint session last night, on pavement, and was happy to find that my achilles tendons weren't whimpering this morning. I think I will try to find a gradually declining grass stretch for the leg-speed sessions, though.
Last night I used a gradual hill that was probably about 300m long and found that it took only about 1:45 (vs. your recommendation of 3 min.) to spring up it, even though I'm trying to make sure that I move my center of gravity more up and down than forward. I wonder what I'm doing wrong? It would be nice if that video came out soon.
P.S. I'm kind of a geezer, so I'm not expecting great improvements in speed; I just hoping that this will strengthen the legs enough to avoid injury during the interval phase.