Most studies that advocate some radical method of training are based on a few untrained athletes over a short period of time. They don't mean much, but they get a lot of attention. Not long ago, there was a study of more than a hundred trained endurance athletes from a variety of sports that compared different training protocols. It last nine weeks, which was as long as they could get athletes to abandon what they were already doing and why several sports were involved. Each group had two weeks of hard training, followed by a recovery week, repeated three times. One group did a version of high intensity interval training, one did a lot of lactate threshold training, one did solely long slow endurance and one did "polarized" training which involved both very long endurance and maximum VO2 uptake interval sessions. The polarized group ended up with the greatest change in VO2max and endurance, which one would expect, as that's what they trained for, but they also improved their lactate threshold more than those who trained specifically for that. It's unknown whether adding specific lactate threshold runs to polarized training would be even better or not.
The training regimen immediately made me think of ultrarunning schedules. Here's what I'd come up with:
T 90 minutes total averaging 75% max. heart rate, with half at lactate threshold
W 20 min warm-up, 4x3min hill at 90-95% max heart rate, 35 minutes cool-down, 4x3min hill, 15 minute cool-down (120 minutes total)
Th repeat Tuesday workout
Sa 240 min trail run at 75% max heart rate, with 6-8x 5 sec. uphill sprint roughly every 20 minutes.
S 150-240 min, done as on Saturday.
T 4x3 min hill
Th 90 min, half at threshold
Sa 120-180 min
To this, I'd add strength training, which I think I described once before on this blog, done as 100 1-meter box jumps. This is the protocol used in studies to generate delayed-onset muscle soreness. I believe that, if one can get to doing those box jumps without DOMS, from training effects, one could probably handle the worst terrain in trail races.