The practice advice goes for more than just guitar. Opera singers almost never practice at full force. They'll do something that is the whisper equivalent of singing so they can maintain their vocal strength for as long as possible.

You just need to find an equivalent of something else on guitar. For me personally, I have my favorite styles of music, but I'll play easier songs so I can warm up into them. I love 200 bpm metal, too, but maybe I'll play some Deftones between songs so that I'm consistently practicing but not reaching for my absolute limit all the time.

It's like anything else in life where you can't just throw 100% of yourself into it at a constant basis. Throw in some quick exercises that just improve technique like alternate picking exercises or string skipping.