You can start any time between 5 and 8 AM - after riding uphill from our cabin to the start (the steepest climb of the day) I got started at about 615AM and arrived in Edgemont 9:35 later. Along the way, they fed us breakfast (pancakes, eggs, sausages, biscuits/gravy, coffee, OJ), lunch (sub sandwiches) and a few aid stations with the usual sugary goodies. At the finish, they had the chicken dinner, ice cream for dessert, and both kinds of beer (Bud and Bud Light).
This is billed as a mountain bike ride, but my teammate Al assured me that the cross bike would be fine, and it was. About 25-30% of the riders were on cross bikes, but there were also a few recumbents and tandems. The trail surface is a fine gravel, a bit looser than most of the hard-packed dirt roads that we ride around here, so 23-width tires wouldn't have worked, although a 28 might do (I was on 32 knobbies at 65 psi).
It's an old RR grade, so it never gets steep. I was able to maintain a steady 10-11mph up all of the climbs, some of which are over 10 miles long. On the other hand, my highest speed for the day was only 21mph, so the downhills aren't very steep.
I took a couple of diversions from the official route, backtracking at one point to make sure that I got to ride ALL of the trail (I'd done the 3 miles from Deadwood to Lead the day before), so I got in about 108 miles for the day, 105 of which were on gravel.
It's a pretty small ride - only 131 people were registered, of whom 109 were doing the full century - so I rode most of it alone. On one stretch south of Custer, I saw only 3 other riders in 12 miles. About 8 miles from the finish, I fell in with 3 other guys on cross bikes who I'd been seeing at each aid station and we rode in together.
The weather cooperated - I started with arm and knee-warmers, but stripped those off by 50 miles. Rain was in the forecast (but never appeared), so I carried a Gore-Tex jacket and a couple of extra tubes, but never needed them.
I'd recommend this ride for anyone looking to do something a bit different. We made a 4-day weekend out of it, but you could do it in 3. Be aware that next year's ride will end in Hot Springs, SD - they alternate finish locations between there and Edgemont.
My pictures here or here.
Two years ago, Al and Sharon did a multi-day tour of the Mickelson Trail, with Al pulling a trailer behind his cross bike. See his write-up here
Big Mick ride website
Mickelson Trail map: Mickelson Trail map
