The high-intensity effort packed into circuit training is an invitation to injury, so a good warm up is essential. And we're not talking about Jane Fonda bouncing stretches. Plan on a ten-minute jog at a good pace, which will effectively move your blood from your core to your extremities. (If you find yourself dizzy at any time during the circuit, especially after a tough exercise that drives blood to the muscles in your legs, take a 30-second break of fast walking to move the blood back to the heart and head.)
McClure's intervals have three levels of intensity, based on increases in time, reps, and levels of difficulty. To find a starting point, run through a full circuit at Level 1. (This should take about one hour including the warm-up.) Remember, proper form is paramount. If you compromise technique, then the speed and strength gains will be limited as well. Once you establish the correct intensity, run the circuit as many as four times (if you've got a few hours to kill, or be killed by).