Today’s game was to climb to the summit on hands and knees: For every step, I had either to “bear walk” or lunge. Though that may sound wretched, it was a huge endorphin fest.
The golden recipe for training is to do as much work as possible while staying as fresh as possible. That’s the closest thing I know to a magical, alchemical formula. Among other benefits (about which more later), it gives you a huge hormonal high.
The trick is to settle in, patiently and quietly, for a slow, long, comfortable session. Think “tortoise,” not “hare.” As soon as your muscles or lungs begin to burn, stop and rest immediately! Let your heart slow down again and get your equanimity back. You’re not in a Rocky training montage. You’re not looking for histrionics, heroism, or anguish here, or even strain. You’re aiming to keep your mood somewhere between “Placid Contemplation” and “Mild Euphoria.”
I’m not saying not to pull sleds and squat logs! 
Just don’t push when you’re tired and grimacing. Throttle back and stay fresh.