I’ll be interested to see others training strategies and experiences and any scientific stuff. I can only speak from my own experience (SS course record at 24HOP, SS colorado trail record, lots of 12 hour mtn bike wins in the SW) as a self coached athlete with a full time job.
I think year over year consistency and a huge base are the biggest factors. Consistency meaning doing something athletic 5-6 days a week, every week, year round for as long as feasible (obviously with maybe a week or two off a year). Lifting, running, hiking, biking, swimming, skiing…whatever, it all helps. Including endurance based stuff on the weekends…5+hr bike rides, spending all day backcountry skiing, 15 mile mountainous hikes, that sort of thing. That inevitably builds a strong aerobic system and body that’s used to being strained all the time.
Then base wise you gotta put in the saddle hours over the winter and 2-4 months out. Riding 5-6 days a week, every week for 15-20hrs, with some weeks into 25-35hr range. Obviously lots of z2, but I’ll generally just let the terrain dictate the effort…most climbing turns into high tempo/sweet spot work, and very few rides are intentionally dialed back to stay below a hr/power number.
Key workout wise, obviously you can’t go out and ride for 18hours to train, as that’s counterproductive. I think doing big back to back days is important though. Back to back hilly centuries, back to back 10k ft days, back to back 6-7 hour days…that sort of thing. When you can do those and you’re still feeling decent at the end of the second day, it’s a good sign. I’ll use a hard 5-6 hour ride 3-4 weeks out from a 12hr race as a good gauge of fitness…figuring if I can do that once at that pace/effort, then with race adrenaline/strategy/fueling I can do two of those. Similar idea for longer stuff…If I can put down two solid and big back to back days, then combining them into one is feasible, physically and mentally. Probably more confidence builders than anything, as a lot of racing at big distance is mental.
I try not to neglect TR and wish I was better at doing more structure, but it’s hard sometimes while still doing huge mileage and lots of big TSS days. Last year I think I had it set on a tri long-course plan, as doing ironman’s was a bigger goal…stayed somewhat consistent with TR through the winter but let it fall off in the spring as riding outside is more fun. Still did great at endurance biking events with that plan. Now I have it set to plan builder, low volume (supplemented with a ton of outside riding) with a “MTB stage race” as my primary event…it seems to have me doing a lot more vo2 max and above threshold work (sustained power build & cross country marathon), which is realistically a big weakness for me right now. Hopefully that’ll work out well after a huge amount of base over the last 3 months and a lot of endurance/ultra endurance training in the past 5 years. Time & results will tell.