David, I’m pretty sure Sean is at least doing an easy jog on the smoother downhill sections, as I know he has trained this way in the past (running a lot with a fully loaded pack) and it would be pretty tough to match his split times without some running – maybe Ralph could do it though! I’m pretty sure Andrew ran a good chunk of the downhills as well.
Personally, on my first attempt in 2015 I found that running the downhills with a full 15 lb pack on the first day flared up my hip flexors, which slowed me down on day 2-3. On my second try I did very little running on the first day and felt much better on day 2 and 3, and although I was a bit slower overall it was not by much. I also didn’t train a whole lot for running with a fully loaded pack, which I think was a mistake.
This is Sean’s 3rd attempt, so I think he has a pretty good idea what he’s capable of and how to pace himself. I’m sure he has gotten a decent amount of acclimation time and lots of vert in his training in the last few months, but as far as I know he doesn’t live at elevation and he has a family so I’m sure his preparation has not been totally ideal. Ideally you would have no job, kids, or other obligations, live at 8-10k feet, run and hike in the Sierra all the time, be fully fat adapted (maybe), have no pre-existing injuries, plenty of thru-hiking experience and 100-200 mile mountain races under your belt, an incredibly supportive spouse or partner with plenty of free time to shuttle you around the mountains and/or train with you, etc.
Ralph’s comments are accurate as well, he has studied this as much as anyone I’m sure. Both Brett and Andrew slept more than they planned to and it worked out well for both of them.