AIBU to want regular gym time while on maternity leave?

As long as you are both getting time to yourselves and time to spend as a family it's fine.

It is perfectly reasonable to have four hours hobby/alone time during the week, so long as your partner also gets four hours of out-of-house hobby time (or has the opportunity to do so, even if he doesn't take it).
If you can't find eight hours in the week to share between you (whilst also finding some whole family time), you will have to settle for half of whatever spare hours you can find.

So he only works and then comes home to be with you and the DC?

I doubt it. But to be honest, even if he does, that's irrelevant - it's totally normal to spend a few hours a wek getting a break and some exercise and I can't see why this would be an issue beyond him being a) controlling b) resentful of looking after dc alone or c) having issues with you doing anything for yourself/working on yoru self esteem and appearance. or d) (most likely) all three.

As a side note, also, many gyms have creches where yo can put the baby in for an hour. I wouldn't do that because of him, but it might give you more flexibility on when you go.

This depends on if he also has 3 or at a push 4 hours a week to do what he wants.

Three or four gym sessions (plus whatever travel time there is and shower time and kit wash time) is quite a lot to find in a week, alongside one full time job and one full time baby plus another child. Does/could your husband manage this for himself alongside work and parenthood if he wanted? I think aiming for 2-3 sessions a week would likely be more realistic than 3-4.

It's really important for both of you to have some time by yourselves, not to mention that being physically active is important. Does your husband get some time too, if not, you both need to factor something in during the week anyway.

Does your gym have a creche.

However, 11 month old babies do not need bfeeding during the night. You'd feel much better if you stopped that.

YANBU. You need and deserve time to yourself and this is a healthy outlet. Do you have much family time together? Can you keep it to three gym sessions a week rather than pushing for a fourth? Does your DH get the equivalent time to himself?

You just do it. Don't pay any attention to the guilt tripping. Men have no idea what it's like, and your DH is an arsehole for not helping you out.

Stand your ground.