How do you deal with the thought of having to work for 40+ years and then when/if you retire, you're too old to do much?

The answer is: do a job you like and do the things you like. I’m due to retire in ten years and won’t be doing so. I won’t work as much but I will work

It’s easier in some countries than others. But the key is not wasting time on things that you feel pressured to do because of societal norms or because you’re too lazy to do anything different.