I did most the Mickelson (Englewood Trailhead to the southern terminus in Edgemont) during my Black Hills Tour. Take a look at the official SD state park's web site. The trail is not flat. Grades of western rail lines are generally steeper than those of the Midwest. The "climb" south from Englewood is several miles. IIRC, it averages around 3%. According to the official site, the hardest uphill section is south from Deadwood. Between Mystic and Hill City there is another uphill section that you will feel.

The other "issue" with the trail is that a lot of it is isolated, meaning your SAG cannot be able to reach you at some points if there is a problem. And towns are infrequent. IIRC, there is nothing between Lead and Rochford, where you will find a bar/restaurant which likely isn't open on Sunday. South from there you won't find anything until Hill City, which has plenty of services. Hill City to Custer has nothing, but it's a short haul with relatively easy access to the highway. Custer has all the touristy services. South of Custer there are no towns until Pringle, which has a bar/restaurant. After that, there is nothing along the trail until the end in Edgemont, where there is a motel and some restaurants and lot and lots of bunnies. (Don't ask me why, but they were all over town.) The cool thing is that there are shelters, bathrooms and water cisterns well spaced along the trail. The locations are shown on the official map. They really got it right.

With all that said, it's a great ride. My favorite section was between Englewood and Hill City. That's where the tunnels are. Hill City to Custer goes right past Crazy Horse. You can see it from the trail. South of Custer has some pretty views of the surrounding area. This album has photos from the trail. You can tell which ones they are. The first one is the one of the gate:

https://www.flickr.com/photos/105349603@N05/albums/72157655263744881

About those gates: There are a lot of them. You have to stop, open the gate and make sure it closes properly behind you. Adds time to the day. So do the many, many bridges. The transition between trail surface and bridge surface was usually not smooth, requiring slowing. In short, budget extra time. Finally, note that you are at elevation in places. Bring foul weather gear in case a storm pops up. During my trip I was hanging out in Hill City one day when two people came into town from the north. They had gotten caught in large thunder storm up on the hill north of town. Matters were made worse by free range cattle that forced them to stop for a while. The husband was not properly prepared for the conditions. His fingers were literally blue from cold. This was in late June, mind you. The first night on the trail, while I was camping in Hill City, I got hit with a thunderstorm with hail. That stone pictured was the size of a golf ball. I think the fact that my tent was under a tree is what saved the fly from being breached.