Unfortunately what is "hard" changes every year, so you need to be a continuous learner.
Trying to negotiate that up-front, though, is hard. Most employers don't like paying much for education and research, and they don't like people who want to work less than full-time, either.
 - not saying use your job time to learn CV-padding stuff; just if you suddenly need to work on a codebase in a language you don't know, don't be afraid to read an actual book instead of minimal amount of StackOverflow answers needed to kill your fist ticket.
If you think that you will eventually burn out, like in “its just a matter of time”, you should seriously consider switching jobs, work in a less demanding industry, or do something else. There is so much more to life than just this work and nothing keeps you from working just at half the pace. You even might get some different insights from that. Seriously, I don’t want to sound like a total jerk but when this is the approach you choose, it’s going to be needlessly rough.
Your username indicates your attitude, btw. Talk to a friend or professional. Maybe you need some good advice.
> you should seriously consider switching jobs, work in a less demanding industry, or do something else
The cognitive dissonance expressed here is...wow. When you have people to support (or even just yourself), you don't just "switch jobs or industries, or do something else" on a whim when you've become burned out. To trivialize the complexity of handling burnout shows a complete lack of understanding what it is like to burn out.
My username is a holdover from an inside joke 20 years ago in my teens, btw. And I don't think I'm the one who needs good advice.
IMHO That's not at all what they're saying. I think they're suggesting making changes long before that, when one hasn't burned out but feels that it is just a matter of time. As far as general suggestion goes (i.e. not directed at you, though it was a reply to you; this is a public forum after all, so you're essentially a stand-in for a wider group of people), I think they're spot on. It may not be relevant to you specifically, since we don't know the details of your situation.
Having seen others burn out in both startups and enterprises, in and out of tech, my opinion is that the advice of the person I responded to is not sound.
I saw a video about Warren Buffet and he used the analogy of a car. What if you could buy any car you liked, but you only got one car for your life. How would you care for it?
Well, you only get one mind and body (so far as I know). I try to take as good care as I would if I only had one car.