The visitor center in Whitehorse is a good place to get that (and other) information, yes. There is a visitor center in Tok, as well, in the same building as the library and they should have information on the road conditions.
100 miles a day is plenty reasonable as an average, though there is a lot of variation in roads and hence actual distances traveled day by day. I averaged considerably more on my trip to Alaska a couple years ago, partly due to constraints, and had a thoroughly marvelous time. I would naturally have loved to have more time, but I also appreciate the benefits of still being employed....