For the last few months I’ve been taking a boxing class in the city. The class is taught by a seasoned instructor known to everyone as “Pat.” Our circuit training consists of the following seemingly easy but tough workouts.Four to six rounds of jumping rope: Trust me — it’s not as easy as it used to be as a kid. I run often, but I underestimated the stamina and coordination needed for jumping rope fast.Four rounds of shadow boxing: You start out with slow jabs and punches into the air. Seems …