Having just driven up to Alaska and back to Iowa, it really does depend on the road conditions and amount of construction, but a 450-500 mile day average is a good 9-10 hours.
Most of the time through Alberta, most of BC, Ontario, you can figure 50MPH average, and easily go 62MPH on the highways if you are not doing much sight-seeing.
Once you get onto the Alaska Hwy at Dawson Creek, there are sections you may only average 30MPH, and others that average 45MPH, and yet others 50MPH. The worse sections are closer to the Alaska border you get. Whitehorse to Alaska, figure being your slowest. Haines Jct to Beaver Creek was the worse segment for us.
The main roads in Alaska are not that bad and most are generally 60 or 65MPH (some slower segments) limits for the main highways between Tok-Fairbanks-Anchorage-Glenallen-Tok. Most of these were in decent shape and you can get a 50MPH average. Glenallen-Tok was a bit slower - 40MPH average due to road conditions.
Construction will slow you down, but if you do searches in Feb at the Department of Transportation of each state/province you can find the planned work for the year. The Alaska 511, Yukon 511, Drive BC, Saskatchewan 511 are great resources just prior and during your trip for daily road conditions, but also long on-going projects.
If you get a chance use Google earth on your route to see the road conditions of when they photo'd the area. Gives a bit of a good idea of the type of roads and some clue what to expect.
Enjoy your drive.