finally been given the green light to starting boxing training

It's true, you probably will, but as you know there are lots of different types of pain especially with this kind of injury. In the middle of my worst back pain I was cataloging them all and I got up to about a dozen. For example, there's achy muscle pain, there's localized stabbing pain, and there's oh-****-I-can't-move-my-legs pain. Since you've boxed before, you likely know your body pretty well. You should be able to tell pretty easily which ones are the "push through it" kind of pain and which ones are the "stop now" kind of pain. Just make sure you keep your trainer up to date on how you're feeling and what your pain level is like - if he knows what he's doing he'll know to stop you when you're in danger of really hurting yourself. Losing the extra weight should help too, which will probably be a natural side effect of training.

Different schools of thought say different stuff about stretching but when you're injured you definitely want to stretch before and after. Flexibility and relaxation are going to be key. If you can budget for it, get a good massage every now and then.

You will likely have days where you can't (or at least shouldn't) do as much as you want. It can be two steps forward and one step back sometimes. Good luck and welcome back!