I'm scheduled to undergo a septoplasty in three weeks. He has advised me to not do strenious workouts for at least three weeks after surgery.
I would ask your Doctor for further clarification. It's possible he just doesn't want you working out to the point where you are breathing heavy. Especially if he did use the word "strenuous". Which I could see being a problem.
I went to a couple of websites of surgeons that performed this operation and all their warnings were the same.
No heavy lifting and no intense exercise as it could cause nose bleeds.
But does that mean you can not still be active?
Walking, for starters, would be a fantastic way to stick with your weight loss schedule. Sure it doesn't burn as many calories as other exercises, but if you walk for the same amount of time you would spend doing those other exercises you won't set yourself up for failure either.
And I'm sure your doctor didn't tell you to stop eating healthy during recovery.
Stick with your original plan with a few minor tweaks.
Nothing about your situation should set you up to have your journey be interrupted. Maybe you won't lose as much weight as you would have without the surgery interruption, that's ok. Maybe you will just maintain your weight while you are recovering, also not bad.
If you continue to eat nutritious, appropriate portion sized food you might even lose MORE weight then you had planned on with your exercise.
Understand that and don't let it be your downfall.