If you’re a chaski runner, odds are you know your way around the route you’ve been running - it’s quite likely you’ve been conscripted for your duties from the local area. Your tambo way station may be a few miles from home, but it’s likely not too far. You only need to pass whatever qhipu messages - or perhaps fresh fish for the Sapa Inca, maybe he asked for some for dinner! - from your tambo to the next one. Then you’ll pass it on relay-race style. You may be one of only a few people around with ready access to the royal road (Qhapaq Ñan) that isn’t military - the road is for imperial business alone.

As for what would happen to a middle-aged chaski, well, it’s likely there’s some land you can be working on - either your own, or otherwise you may be asked to fulfill your mita labor service to the empire by working on the royal estates or performing local maintenance on those way stations and roads I just mentioned. Food and security will be covered by the Inca from storehouses you pay into, and they’ll very possibly be quite sensitive to the needs of the household and community. Which is to say, if you’re the only male head of household the Inca will find other ways to get their mita, or else they’ll only conscript you when your fields don’t need a lot of attention. Planting and harvesting are both moments of “all hands on deck” for a village. Hopefully your community was...amicably brought into the Tawantinsuyu. But of course, if you were sufficiently recalcitrant, there’s a good chance you’d never see your family or friends again, having been completely atomized and sent to far corners of the empire to act as migrant workers on royal estates.

Now, as you get very old, you may end up taking on a job traditionally reserved for women - weaving. Hard for you to pass the chaqitaqlla (digging plow) into the ground, but you can drop spindle some thread for your wife or niece, and maybe even get passable at working the loom. Of course, there’s likely all manner of political negotiation and labor distribution you could while away your time with, too.

EDIT for sources: Okay, now that I'm awake, caffeinated, and questions have been answered, here's a couple good sources on Inca infrastructure:

John Hyslop's 1984 The Inca Road System from Academic Press of New York, and his follow-up in 1990 Inka Settlement Planning from University of Texas Press, are both excellent resources. Hyslop followed many of the old roads (traveling on a motorcycle, natch) and investigated/reviewed the chronicles to confirm.

John Rowe, in 1946 wrote for Julian Steward's Handbook of South American Indians with a chapter of about 150 pages called "Inca Culture at the Time of the Spanish Conquest". It is a classic, and still quite insightful for those curious about the ins and outs of Inca practices.

In another comment below, I mentioned a few good baseline resources to tackle Andean studies in general, and the Incas in particular. There's plenty more if you have particular questions!

Today the Qhapaq Ñan is UNESCO world heritage, stretching across six countries (Chile, Argentina, Bolivia, Peru, Ecuador, and a tiny tip in Colombia) - there are more reports and information out there too. Happy to vet any sources that crop up in your own search for more information!