I was looking for a good long UTMB training run and stumbled upon the Upper Big Cottonwood Traverse. The distance and elevation profile were ideal, so I thought I would give it a go.
I drove myself to the trailhead and started up the Mill D N Fork Trail just before 9am. I was carrying about 1,000 calories in the form of liquid, gels, and bars, and about 3 liters of water in six 500ml flasks shoved in various pockets of my vest. I had no intention of stopping to refill my water and hoped that the day would not be too hot, allowing me to move fast enough to survive on only the water I was carrying.
With the majority of the vert occurring in the first 5 miles, I figured I would push a little harder during the first bit, reaching the shoulder of the Wasatch Crest Trail in just under an hour. The trail was pretty cruiser along the crest until Guardsman. There were a ton of mountain bikers coming the other way, which definitely affected the flow. From Guardsman to Brighton over Clayton was not super runnable. Once I reached Brighton, I realized that the trail descended into the resort. I think it would have been really cool if the route stayed on the ridge and went out and around Wolverine Cirque (possible variation).
The rest of the trail is really runnable. I did make a few wrong turns, which I had to double back on. I would suggest making edits to the GPX file to avoid the same wrong turns.
All in all, this route could be done FAST, and I don't expect my time to stand for too long. This route can certainly be completed in under 4 hours and, with the right fitness and conditions, I anticipate someone taking this deep into the low 3-hour range.