Sounds like a very fun race. Boulder has a long history of this kind of thing. In the early 70s there was a race from the dining hall in Chautauqua park (making a loop around the paved road) to the summit of Green Mountain anyway you wanted to go (not mandatory to sue a trail). Then as now Boulder was home to several world class runners including freshly minted Olympic gold medalist Frank Shorter who was a a 2:09 marathoner! These sub 28 minute 10k runners would set a blistering pace on the flats that the rest of us could not begin to match. But the winner was always some Norwegian Cross Country skier from CU. Quadrupedal propulsion was the mode for steep scrambling.
The flatirons are even steeper than the rough 3 class terrain we chose but the idea is the same: Besides the moderate technical skills you need to be well trained aerobically with all 4 limbs. Not much better way to train that then accumulating a big volume aerobic work using all 4 limbs before adding intensity. Classic XC skiing and Skimo mimic the demands for the upper body because you are using poles which put your body posture in a very similar position as when on steep terrain scrambling. Classic XC uses the upper body even more than Skimo because the grip of the XC skis is much more tentative than full length skins.
While I was working with Steve in the early 2000s he did a lot of 3rd class scrambling for aerobic base training.
As with all high intensity endurance events the aerobic base is THE key to success. Focus on developing as big an aerobic base with all 4 limbs first then apply very event specific high intensity workouts on top of that base. Don’t leave home for a run in the mountains without poles in your hands. The ski striding will do what you want for much of the base training. The advanced Skimo plan would provide a good template for this approach. Be aware that the bigger the base the higher will be the peak when you apply those Flatiron interval sessions.
Neal Beidleman and Kevin Cooney used to put on the Basic Boulder Mountain Marathon until the park police shut it down in 1992 as I recall. I have that last year’s t-shirt still. It started and ended in Chautauqua but went over the skyline to Eldo and back. It got too big.