Sometimes when I'm running, I think of myself as a machine that takes fixed-size steps at a fixed rate. When I'm going uphill, the machine loses charge. When I'm going downhill, the machine regains charge. When going downhill, there's a tendency to run as fast as you can. If you do that, you will not recharge, and the next uphill will wreck you.
There's an analogy here to working pace. It's better to make steady progress than theoretical max progress. On easy days, life is easy. Let it be easy. Resist the urge to overachieve. Quit early, have fun, be yourself. Charge your "fun" reserves. These reserves get you through the hard days.