My questions is: how advisable or unadvisable is it to climb Fuji without staying at one of the mountain huts?

I am in japan, part of my travels around East Asia, and climbing Fuji is one of the things I've looked forward to the most. And I'm intending to do it over the next few days if possible. However, as I'm on a pretty tight budget to say the least, I don't wanna pay to sleep in a mountain hut for a few hours. And I'm guessing that at this stage the huts may be all booked out anyway. Can I simply walk straight up from the 5th station at night? If it helps, I'm 25 years old and in good shape, do a lot or running etc and have climbed highers mountains that Mt Fuji before (although that was with guides).

Not a problem, hiking the mountain without stopping anywhere overnight is probably the most popular way of climbing Fuji. The only thing to consider is your timing if you intend to see the sunrise. It takes 8-9 hours to climb at a moderate pace so you would want to leave around 9-10 at night depending on how fast you hike. Also, be sure to bring warm clothes if you do the overnight hike.

Start from 5th station early, try catch the first run bus.

Monitor yourself for HAPE OR HACE.