This is a variation on the Elk Point/Jefferson loop. Coming West out of Jefferson you will hit a paved road in 2-3 miles that you can go South on. This will take you into McCook Lake/North Sioux City. I usually take this to the Casey's General Store and turn around and come back the same way. Upon hitting the point where you initially turned south, go back west to Elk Point.
To turn this into a (near) 75 mile ride I also rode a small little loop on the way home from Elk Point. A mile west of Elk Point is a paved road that drops down and comes back around to the Burbank road.
As you can see by the profile, it's a super flat and fast course. The only hill is coming back into Vermillion.