Basically you can do Alaska either in a clockwise direction or an anti clockwise direction. besides a few side roads there really are very few choices to make.
Same with getting there & coming back. You can do the Alaska Hwy up & the Cassiar back or do it the other way round.
As far as speed goes, well, it depends. You can get very long stretches where 50mph can be done quite comfortably. Alaskan highways are very good & often 60mph can be the norm. You can also get stretches where work is being done & a convoy leader will take you through. No telling what speeds will be for those sections. Sometimes the wait is long enough to get out & get to know who is infront & who is behind you.
If you think that you can blast through & do in three weeks what needs months, think again. Alaska is not that big. Canada is. Canada is much bigger than most think. Canada also has some of the best sights to see.
Also, those side trips to places like Seward, Homer, Wittier, Haines, Hyder & others, take time