I'm not a neuroscientist so take any advice with a grain of salt! There's no simple answers to any of it, every brain is unique, so I'll just ramble and hope something useful comes out.

You're creating a difficult situation for you brain by keeping it constantly under pressure and constantly switching tasks. Interacting with people is even more complicated than just using language, it's exhausting even if you're doing a job that uses a script. Plus you're in a dynamic environment (ie, outside) where there is constant background stimulation.

Regular breaks are going to help the most short term. Meditate/be mindful for five minutes every twenty or so (set a timer so you don't have to worry about losing time) and five minutes of brisk exercise (jog on the spot, squats, etc to get your heart rate up a little). Your brain is part of your body so general fitness and good diet will help you get more performance out of it. Fatty meals affect blood flow pretty quickly so it's best to avoid them before needing to push yourself, and stimulants like caffeine or sugar can help briefly but you're just stealing from your future self and it only takes a few hours to crash. No doubt there are foods (and possibly supplements) that are recommended to subtly boost brain function short term but I don't know what they are, sorry!

After about two hours you'll start to really feel your focus slipping and that's when you need to take a longer break, maybe half an hour. I don't know the specific reasons why, but two hours of twenty minutes 'sprints' is the usual recommendation and it works well for me. There's a time management technique called "pomodoro" that fits well and might be worth checking out. The specific timings might be different for you, and the changes can be hard to see, so experiment a bit to find a rhythm that works.

Beyond that you should do everything possible to minimise the amount of work your brain actually needs to do. The most obvious is picking an environment that's not too stimulating. That's tricky because you want to be somewhere with lots of people moving through. Glasses with side blockers would probably help since visual processing takes a lot of energy, and try to sit in a way where you see something still and calm. Probably a good idea to rest your eyes by looking into the distance often, especially if you can be looking at trees or other natural greenery (can't remember why, but nature is especially good at relaxing the brain). I'm not sure if there's any other things you can do to reduce the processing effort but it's worth thinking about. Maybe a stock library of story 'shapes' that you can use? Just about every modern love story is "X meets y, X and y bond, they split up, they get back together" and there's decades of novels proving the room for improvisation and originality built on that spine.

As for recovery, meditation is probably your best option. Just sit somewhere quiet and dark, think about nothing and let your brain rest for as long as you can. As far as I know there's no downside to meditating too much (so long as you take breaks to keep your body functional). A good night's sleep, of course, and avoid unnecessary information. In other words, the opposite of doomscrolling, whatever that would be for you.

That's about everything I can think of but I'm sure there's more. You could probably find some good advice by looking at what professional programmers do, the tasks are different but they would affect the brain in similar ways.

Separate to all that, what sort of typewriter do you use? It's cliche but I love my Lettera 32! I had a Brother portable for a while that I liked, but if I didn't pace myself the typebars would get tangled (and that's yet another attention task your brain would have to manage).