Tire makes me tired

Gotta love David and his creative ways.
Today halfway through class, he informs me that I'm going to be doing 50 stepups onto the tire. Feeling confident I said
"I think I can do that. I did 111 on my chair last week and that's the same height"
The second the words left my mouth, I wanted to eat them.
I knew I was in trouble.
"Oh yeah? Then you can do that 50 with a 20 pound Kettlebell"
"$%#$^$%&%&%"
So I did and MAN did I struggle.
I did the 50 and when I was at 40 he tacked another 10 on so I did 60. At some point and I'm not sure when I was struggling HARDCORE so I switched to a 10 pound Kettlebell
I only caught a little bit of it on video beacuse the battery in my camera died.
My legs are currently aching like nobody's business.
I just now decided that I'm going to do it again on Monday and every day I can before the Climb to the Top event.