I went up last year from the hwy. The gate was closed and we planned on driving up top. About 5/6ths of the way up the hill cars started to come up lol. The base of Fortress is about 7000' I believe and Id say the highway is probably around 3500-4500'. Thats my guess, so its a ways. Its a long way in. Id suggest if they have been grading it and the gate is closed, or if there isn't much snow in that area which is usual you could ride a bike up top with your skis on your back, then your ride out is quick aswell.
About 6 years ago there was one of the pepole who owned it driving around in his cat up there. I don't know if he still does it. He had a rope hanign off the back and if you were lucky enough you could hitch a ride.
Have a good time up there, super nice area and great steep lines to ski. Have a break in some of the cool wind formations around the old lodge. Pretty neat.
Terry