I would say that it is too ambitious, but it is what it is. Also wish you were not staying in Keystone, but that too is what it is. This is the route that i would suggest: Deadwood Crazy Horse (this was the least interesting to us, you can see it from the highway if you wish) Go to Custer, take wildlife loop west to east. Visitor Center for Custer SP Take 16A west to 87, take 87 (Needles HWY) to Sylvan Lake Take 87 back to 385, make right turn onto 244 to Mount Rushmore From Mt Rushmore take 244 to 16A (Iron Mt Highway). Turn around in the vicinity of the playhouse rd area and drive 16A back to 244, and you will be a stones throw from Keystone. I'm not sure how many hours and miles it is, but that will optimize your day to the fullest.

We took around 5 hours. Mainly because of stoppages to watch the wildlife. There are many places where these are available at arm's length and is worth to wait and watch them leisure. There are places for small parking places by the side of the road.