Pacing is key. I think the best way to talk about pacing is to compare it with running. If you’ve ever tried running a distance with a time goal in mind (say, run 5 kilometers in 20 minutes), then you’ll need to stick to a strict pace.
You don’t want to run too fast so that you lose strength and focus midway. But you also don’t want to go about it too slowly.
Learn your natural working pace. Ask yourself:
“How much productive work can I achieve in a day?”
Then pace yourself accordingly. You’ll know whether you’re doing too much or too little in a day.