The doctor is telling you he will write in whatever answers you tell him to because only YOU know how long you can sit, stand, etc......and you are complaining ? He can't WATCH you for a full 8 hours (in a typical workday) to see how you function so he is asking you how long you can do things.

You don't have to be a doctor to complete the answers. You have to know what you can do.....and for how long. YOU are the only person who knows that.

That sounds like the perfect situation to me.