mrsnowman, you've had surgery and a UTI so whatever your brain might be telling you, your body is saying
"Come on mate, give us a break!"
Just because others (who may well not have the kind of heavy duty job you have) made it back to work in a few weeks doesn't mean that you should.
Two weeks ago is nothing, especially for a physical job so give yourself some time. If the surgeon told you 3-4 months then that's how you should be thinking too. It's not skiving to take time off sick.
What about a compromise. Tell yourself you'll review your situation in 4 months time and make a decision then. Would it be possible to take on jobs with lighter delivery loads when you do return?
What would be the point of having life saving surgery if you risk the quality of that life by giving yourself a hernia, or worse.
There is no need to feel guilty. You've earned the right to take time off.