I think Unit 100 may be one of the easiest elk hunts in America. The last time we hunted antelope in Unit 92 (same area as elk Unit 100) we saw elk up by Oregon Buttes in the northern part of the unit. There are elk all over in the slightly "rougher" broken country after the shooting starts (all of it is pretty easy terrain). You definitely won't have to get "a few miles" off the road, just look for pockets in the more broken country where you can't see into it from the road, and by that I mean maybe a quarter or half mile away off the road. Find high points and use your glass, and you will find elk.
I love having an ATV for a hunt but I certainly don't think it would be necessary if it's a pain to bring with you. There are lots of roads.
We drew two tags many years ago and killed two bulls on the first two days of the season. We had done virtually no scouting or research for the hunt, and just showed up opening day "blind". There is a reason even the bull hunt runs around a 90% success rate.
I am not the best guy to ask about when to hunt the unit, but I recall it is only a two week season, so it's not like the month long seasons you so often run into in Wyoming, where you can hunt a couple weeks after the opening crunch.