So, I left my adventurer training for about 18 months in my fortress while I built a keep. He made good progress. I had him train under three different teachers: wrestling, mace, and axe. He got competent in striking and mace, skillful in dodging, etc, etc -- all from nothing. I also equipped him with 2 artifacts: an artifact leather helm and an artifact rope reed skirt (hey, it was menacing with spikes of iron...). Interestingly, when I retired the fortress and unretired the adventurer, he only had the leather helm -- and he was hauling that. Furthermore, he had completely different equipment from when I retired the fortress. So assigning equipment is hit or miss. Still, he had no trouble wearing it and leaving.
I also tried to recruit my mates in the fortress with success. But since he still had no reputation, I could only convince recruits to come along.
I've retired this fortress 2 times before to have an adventure -- though this was the first time I tried this trick. In the 2 years since the last adventurer, the world has gotten much more full of goblins. There are dark pits completely surrounding my fortress now. Looks like I'm in for some fun. I tried to scout out one of them, thinking I would have no trouble. Wow was I wrong. These guys are heavily armed and highly skilled. I didn't even land a single hit.
Good thing I built the keep. And... man are they going to be pissed about the artifact leather helm...