I still get up to my club at least once a week, I work 12-9 quite often during the week though.
Its a great way to get fit, although if you want to compete it takes a lot of commitment. Not just to get fit but to get your technique and timing good enough.
In a decent club they will let you join in their training session (usually go for a run or sprints or something, bag work, circuits, a bit of sparring or technique work). All they will expect is you give your best, and they will probably give you some one on one work to get your stance and guard sorted, then as time goes on get you doing some pad work. It comes with time and practice.
I had two fights when I was 17, then my grandad went into hospital and various things happened and I went to uni.
Sparring is a must if you go to a boxing gym, you will only really learn when you get in the ring with somebody. I'll be honest, I'm a big lump and I'm not too bad (used to spar with three England internationals who are now pro's) even though I'd been out of it for years but its natural to be scared.
Its how you deal with it that matters.