She could quite easily have retired earlier. Maybe she enjoyed what she did. We all have constraints aswell, things we have to do that bore us.....shopping, ironing, filling the car with petrol, making small talk, tolerating in laws etc.
She took an oath to serve and she adhered to it, that's admirable. The boring constraints you've listed can't be compared to that of the Queen. If I can't be assed filling up my car I won't. No one will care. If the Queen couldn't be bothered to meeting a bunch of old ladies at the WI in Southend that's a different matter.