You is from Phoenix, Arizona, eh?
Three things change in Alaska, the Yukon, and Northern BC.
1. The temperatures, 2. The amount of daylight, and 3. Precipitation.
The temps start to drop quickly - more so in the interior -
Highs in the 40s F, lows at or below freezing.
http://www.climate.weatheroffice.ec....s=&StnId=1617&
But September is the rainiest month in southeast Alaska - torrential.
Almost 8 inches in Juneau in both Sept and Oct.
http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?ak4100
Plus you are losing about 5-6 minutes of daylight per day.
http://aa.usno.navy.mil/cgi-bin/aa_rstablew.pl
I live in the Northern Rockies and have cycled/hiked extensively in Wyo, Mont and southern Canada in the fall - plus up to the Arctic in summer. Fall is a time of great beauty, but also of rapid and dangerous weather changes. My friend who works in search and rescue comments that they always lose a few hunters in the mountains here in fall. It's far more isolated and extreme along in the Yukon and northern BC - plus many of the roadhouses have closed for the season. So what may be 40 miles between help in the summer may be 120 miles by October. Granted that there is a good deal of traffic on the Alaska Highway - far, far less on the Stuart Cassiar.
If you were to do this, I would urge extreme caution and the willingness both to wait a few days here and there - plus the willingness to hop on Greyhound, if necessary. I'm just guessing, but I don't think you have much northern winter experience. Being on a bike in a blizzard in the middle of nowhere is not the way to get it.
Best - J