$800k does seem a little low, but a lot of that depends on your living situation. Even with $800k you should be able to live off of way more than $20k/year. You can get $20k/year just by buying treasury bonds and collecting interest, maybe even double that potential income if you think that social security will still exist in its current form when you're retire.

More importantly, once you're in your 80s it's probably safe to start whittling away at that principle.