The phrase "hour marathon" means doing an activity for a full hour, without stopping, just like a marathon race, which usually lasts many hours. Full definition
It also makes the notion suggested by some that we will see a 2 hour marathon in the next 20 or so years look increasingly ridiculous. (athleticsweekly.com)
Three hour marathon episodes too long for your busy schedule? (tech-gaming.com)
As regards a sub 2 hour marathon in 18 to 20 years — dream on. (athleticsweekly.com)