Not only the grade but the surface. I picked up the Mickelson at the Englewood Trailhead after riding up Spearfish Canyon. While I missed the most difficult uphill section, I was immediately faced with a more than 4 miles uphill section that rose about 650' according to Ride With GPS. And it maxes out at over 6,200' in altitude. That, the surface and a slight headwind was really tiring. Later in the evening, during the climb up from Mystic, I came across a group of kids with a chaperone. They had started in Deadwood and were headed to Hill City. They were so wiped they were walking. I later felt concerned for them because I bombed the downhill to Hilly City and ended riding to camp in the dark after a quick stop at the market. So I know there was no way they made it to town before nightfall. However, I am pretty sure they had enough time to make it to town before the massive thunder/hailstorm hit a few hours later.
Those storms can be dangerous. Later during my trip I met a couple in Custer who was getting a shuttle to a point north on the trail with the plan of riding back to Custer. I was headed to Hill City and back for the day and told them I might see them there. Well...I did. It was raining lightly in Hill City so I was hanging out under a trail shelter. The temperature in town had dropped some 15 degrees when the system moved in. The couple pulled in to the shelter after having been caught in a bad storm on the trail north of Hill City. To make matters worse, they had been forced to stop for a while thanks to some stubborn, free range cattle blocking the trail. The wife was properly dressed, but the husband was not. He was shivering and his fingers were literally blue. He called the shuttle service, which was based in Hill City, and they got a ride back to Custer.
Never ridden the Paul Bunyan. Not sure it existed when I crossed MN many years ago.