I wish I could give some advise about the job. All I can say is go at the pace you can, not at others pace. If that makes sense. Try to schedual alot of sitting breaks, if you can.
I recently went on a trip for 2 and half weeks. I went to my doctors the week before I left and she/they helped me to plann out my trip. Meds needed, medical supplies, pharmacies "Just in case" and emergency contact numbers. I was apprehensive at first, and frankly afraid to bring it up. But once I did, I was glad and greatful for the help. So, my only suggestion is to talk with your doctors and plann it out. It makes things so much easier.
I really hope you are comfortable with whatever you decide. It sounds like you are really excited about the job for the most part, so maybe you and your employer can come to some sort of compramise.