I haven't biked that route, but I have driven more or less that exact route in mid-September (driving back from Glennallen, Alaska, down to Seattle). Mid-September you might be looking at lows in the teens or 20s at night in the interior, and yes, the possibility of snow. It's a pretty time of year, though, with fall foliage and the like. Bears may be more active since they are frantically bulking up for winter, so be extra-extra bear safe (which I guess doesn't make sense, as you are either bear safe or not). 70 miles per day might be optimistic, especially if you run into any construction zones. On a plus note, you'll still have 12-14 hours of daylight in Alaska in min-September, so you have good time to bike.
The bigger concern is that in September a lot of travel-oriented businesses start shutting down, so you can't always count on a business noted in the Milepost being open.
Here's a CGOAB journal from some folks who biked in the Yukon and BC in the fall. Might be useful:
https://www.crazyguyonabike.com/doc/...id=12974&v=1Gu
By the way, from Anchorage you'll ride more east/NE to Tok than north ;-)