I'm at the boxing gym 4-5 days a week, and at the regular gym the other times.
All i can say is run. If you can't run, you can't box. It's alright saying 'i can punch' - not good enough, you need to be able to punch for rounds.
You should spar everytime you have a chance too. Nail your technique through the heavy bag and a coach.
This is pretty much my routine.
M: conditioning - core training - skill (boxing) training
T: strength - skill
W: conditioning - core training - skill (boxing) training
T: skill
F: conditioning - core training - skill (boxing) training
S: strength
S: rest (active)
With another weight session thrown in when i feel my best.
core training is just that, abs/planks, medicine ball work, if you're stuck i can post loads.
skill training is sparring, heavy bag, speed bag, technique, speed work, calisthenics.
conditioning is running, HIIT, hill work, calisthenics/bw.
strength is weights, lift explosively, compound movements. don't forget the legs, weak legs = weak punch.