Winded =/= gassed.
Gassed, to me, is my body beginning to shut down.
Winded just means that I'm breathing really, really hard. In a really well developed cardiovascular system, like Mitchell obviously has, he can go at that rate of high exertion for a long, long time without producing enough waste in his muscles to result in degraded performance. Basically, he's trained his body to remove and process those byproducts of aerobic respiration at a very high rate.
A while back, DT mentioned doing an FTP test on his bike. I was kind of surprised that it was as low as it was, given his size and the fact that he crushes a lot of his opponents with his activity level. It would be interesting to see how Mitchell would do at the same test. I wouldn't be surprised if it was higher in terms of both watts/kg as well as raw wattage.