I suspect this is your 1st adventure through Canada to Alaska. You'll have no problem getting to Canada at 600 miles per day but once you hit northern BC and all of the Yukon you will spend the better part of 24 hrs a day to cover 600 miles per day. Between Frost Heaves, constructions zones with crappy roads and long delays you better leave at the crack of dawn and plan on driving till dark, noting night driving is not in my wheel house with the many animals, moose, bison, goats/sheep, caribou etc that hang out on the highway, especially at night.
Having lived in Alaska for 25+ years and transiting the Alaska Hwy dozens of times by the time you head south you will experience snow on your way home in September in places.
Just like RV'ers who leave too early each spring because it's spring in the Lower 48 they end up stalling out on the trip north because it's still winter up north. Most RV parks don't open till Spring arrives up north which changes from year to year. Same with fall closures. When the snow and cold arrives they shut down, provincial and private parks.
Good luck.