I am less worried about not being able to do my job and more worried about age discrimination. I am definitely in a field where you just age out - me now probably wouldn't hire me then. Not for age per se, but rather things that come with age. I get it, it sucks.
I do think I can find other things to do, however. Maybe I can't administrate networks forever, but I'd like to think I could work at CarMax, work for a company that does application, training, or - if I get off my ass - earn a little income doing some sort of craft work (sadly, not Kraftwerk). Woodworking (and techno) has always interested me, and I would enjoy a future like my friend Alvero who builds high-end standalone cabinetry (like armoires) to supplement his income. He makes a few of them a year at $10k each, half of which is his labor. An extra $15k on top of SS and 401k would be great - especially because it's an active and productive time-filler.
There are definitely some jobs you can't do forever - you're not going to actively be building houses at 75, but you have to prep yourself to take on alternative roles. The company we primarily use for home improvements employs some of their aged laborers as estimators. That's a great job for a 75 year old where experience is invaluable. I keep my eyes out for these things, because I know I can't do this forever *and* I don't want to be a sit around and do nothing retiree. I wonder what late-life career opportunities are at burningman.org?