Only reason I've ever used "average" speed in AK is to build lead time in for a specific activity at a specific location: getting the dipnet charter at Chitina at 0900...better leave at 0300 (i.e. 300 miles, 6 hours @50mph); meeting the BIL at Squarebanks 360 miles away requires just a bit over 7 hours .....and so forth.
If taking a road (or air) trip around AK just for fun, about the only concern for speed is "I have to be back in Los Anchorage in XX days"...and given that this is the road system in AK, one is never more than 1 - 3 days drive from Los Anchorage depending on how hard you push it and the time you build in for contingency. There has been more than one time when I was 20 miles from a road and weathered in in the PA-12 for 2-3 days in a row....if I was able to get to that road, it would have been a few hours drive back to Los Anchorage.