I am interested to see how a few factors play into Randy's cardio...
Age... Time off... and Lesnar's size
Sure Brock weights in at around the same weight as Big Timmeh, but he also rehydrates up from there a significant ammount of weight. Mike Goldberg is the one who said Brock fought at 280.
But the big thing is, weight isnt the only thing to take into account in his opponent. The force behind that weight is just as important, with Lesnars strength, plus wrestling skill I am just guessing clinching / grappling with Lesnar would be more tiring that even a 280 lb Timmeh