8K calories is on the high end of what Tour de France riders burn on the longest stages. It's basically doing 5+ hours of effort putting out power that very few people are even capable of for 20 minutes. Or something like 8-10 hours of lower intensity exercise (lower intensity being relative of course, we're still talking 6-700 calories an hour).
Cycling is a good because with a power meter you can precisely measure the watts and multiply that by normal human efficiency (usually 24-25% with outliers on either side) to gauge calories burned. So a Tour de France rider doing 300 watts for 5 hours is about 5,500 calories, plus whatever they burn the rest of the day. 8K is possible, but it's pretty extreme for non-elite athlete. I'd assume that that figure is just an overestimate of a still very extremely strenuous day.
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