I think the 400 mile maximum day is correct, more or less in Northern BC, Yukon and AK. I would spend less time in the cities, unless there is a reason. For example, Homer is much more interesting than Anchorage. Also, there is always the issue of road construction. The Canadians and Alaskans will irregularly tear up and rebuild up to 30 mile segments (more likely 10 miles pieces) of road, leaving you with a wide variety of surfaces to navigate. I have run into blasting work, that has closed roads for an hour.
There is a saying among AK travelers that is very, very true: "Never believe any advice on road conditions that comes from travelers who haven't been over the road in the past 24 hours." It's true. Things happen that fast. Be alert for any change in color or texture of the road ahead. Slow down for them. They could be unrepaired frost heaves that can throw you off your bike. They can be the beginning of construction zones, where there's NO PAVEMENT. Hit one of those at touring speed, and you're going home on an airplane. If it has just rained, unpaved segments can be treacherous. After a day or two of dry weather, they might be good, plain fun.
Don't underestimate the scenic beauty of Yukon and BC. I think many riders find these roads just as good as (or even better than) the Alaskan roads. Study your Milepost carefully. There are lakes to camp on and hot springs to soak in that are real high points of the journey. The BC road 37A, which tees off from the Cassiar to Hyder, is one of the most beautiful roads I've even been on, bar none.
Haines is a nice visit, especially the Eagle refuge, where you will see more Bald Eagles in five minutes than in the rest of your lifetime. You can take your bike from Whittier to Valdez on a wonderful ferry ride past a calving glacier. From Skagway, the train ride is well worth the time.
I think those who have been to AK can close their eyes, and be right back there. It's a pretty nice ride.