Lets think about this a little differently:
Its your first day back to work after a surgery.
30 minutes of walking is great. An hour is the goal but 30 minutes is wonderful.
It is okay if you dont make the full hour every single day. Guilt is not helpful when you are still recovering from a surgery.
Another positive is that you are not drinking.
I say, you are killing it today! Keep up the awesome work. Have a restful guilt free night.