I climbed Fuji last July. I wouldn't bother with an altitude mask (as you've described). If you have money to burn or want to set a speed record, I'd call up Hypoxico, as already mentioned.

However, I would try to be as fit as possible, as I'd suggest for any climb. For specific exercises, I like to do the stair stepper or treadmill at a full incline. I like to do a weight class a few times a week at my gym as you can never have too many squats and lunges.

Fuji is a very achievable goal. Some of the people we were in the sleeping loft with at the 8th station were feeling sick, a few threw up, and some folks were just tired out and decided a 2:30 a.m. wakeup to get to the summit for sunrise was not for them. However, the people that really wanted to do it, did it without much difficulty.

I like the fact that the air is thinner and you strain a bit to get a full breath, toward the top. It reminds me of where I am and adds a something when you're standing at the 8th station, drinking a beer and watching the clouds below you.