Speaking to BBC News from his eighth-hour treadmill, Mills was in high spirits, thanks to the combined effects of a five-minute break and a giant donut.
“I just went out for the first time and it was really great,” he said. “I’m going to do it more often, because I forgot there was an outside world.
“Here in the studio, it’s like Vegas – it can be any time of the day.
“It’s gray and damp and cold outside, but it’s not here, so the change of scenery was awesome.”
However, he admitted that he was starting to get tired.
“My words are coming out and I’m not entirely sure what I’m saying. But I’m still walking.
“The delirium is going to set in around 4 a.m. Then when I see Zoe Ball on her breakfast show tomorrow morning, I’m going to be like, ‘OK, we’re fine. We don’t have a lot of time ahead of us. .””
Her co-presenter Jeremy Vine had other concerns, however.
“If the bosses see this,” he joked, “they’ll arrange for us to power the whole station while we broadcast.”