You can't pour from an empty cup.
Exactly. You won't be able to care for anyone if you're burnt out, and presumably you've got several weeks ahead where he will need care after this op?
On top of however long you have already been his 24/7 carer. Plus everything you had to keep doing alongside that.
As I read it, you were just trying to manage his expectations about where you may be when/if he finally gets the OK to leave and that you may therefore not be instantly there.
You weren't saying after he gets the OK to leave you're going to go off and do X, y, and z. Although frankly, going for a walk first before you resume being his carer is not a big deal (unless it's some kind of epic trek...). You need respite.
Being discharged is not an emergency, you shouldn't have to drop everything to rush to him, and nor should you have to cancel your whole day just in case he's discharged. He may not be!
Other people have to take transport home alone and manage alone. He's incredibly lucky to have had you there to do so much for him, and is being selfish and ungrateful to expect you to be there at the click of his fingers for a non-emergency, not even urgent situation.
Don't martyr yourself. You're a human not a robot, and you need taking care of too.