It's On the Map!!
Sitting at the Hubbell Canyon Trailhead, looking across Forest Road 64, you will notice a large pile of rock. It is not natural.
It blocks an old logging road!
This route is on the topo map, and it does end in less than a mile, just like the map shows. The map also shows yet another trail nearly intersecting that old road and heading down the hill to the Sacramento Basin. I have yet to find that second route (I do not believe it is there!) but the old logging road kicks off a great off trail adventure. At it's end, follow the bench down and around to the south. Do not drop below the natural bench, but continue to circle around the hill, heading for the saddle around the 9,000' level. From there, you can find an overgrown elk path that drops steeply into the canyon immediately to the south (second over from the Sacramento Lookout Peak).
This drainage makes for a nice hike to the bottom of the Sacramento Basin. It obviously was harshly hit by a fire, burning away much of the larger growth. The resultant re-growth accounts for the heavy scrub in the top of the canyon. You could try to make your way down from that upper point, but this is a better route.
Once at the bottom I believe you will enjoy a much nicer return hike back up Thousand Mile Trail. Walk up the paved road to the north, to the signed trailhead.
This is a very overgrown canyon. There are somewhat clear paths if you are lucky enough to find them.