I feel pressured to go back to work after having surgery?

If I am reading this properly they still want you to come in but you will be doing administrative duties. This is usually giving orders and processing paperwork. Nothing standing or difficult. Every company except the most flexible ones will want you to do something. The reason for this is that they have a lot of work to do to function and if you are hired you might as well be productive even with the injury. It is standard practice to pressure you. The best option would be to make sure you are not pushing yourself too much and consult the doctor as well. In the case of knee pain it might be wise to see the doctor first and the supervisor later since you might have a complication or slow healing from the surgery. If you go to work you might ruin your knee.