How to Get Better at This?

Monday morning = another 5:45 am H.I.I.T. class.  I already blogged about how last Thursday, Marta worked us so hard I thought I might puke in class, so today, I was expecting more of the same.  She did not disappoint.  Today, Marta had us do a circuit.  We did the following exercises:

     *20 star jumps (jumping jacks with our legs and arms flying out)
     *5 clean-ups with a kettlebell on each arm

     short rest

     *20 tuck jumps
     *15 clean and presses with a 30-lb bar

     short rest

     *20 box jumps
     *10 burpees

     short rest

     *20 plyometric lunges with hands above head
     *10 bicep curls with the 30-lb bar
 
I think we did that full circuit four times.  The last two times, I was absolutely failing.  By the time we'd get to box jumps, I was so out of breath I was stopping every couple reps.  I know it was by no means easy for any of the other four women in the room, but I felt way more out of shape than they seemed to be.  

So, how do I get better at this?  Besides attending my H.I.I.T. classes regularly, of course, what can I do to increase my capacity to work harder for longer periods of time?