I'm pretty sure Levins has been training with a weight vest:
"Levins, who said he had previously run about 120 to 130 miles a week, has been sworn to secrecy about the particulars of his training regime by his college coach, Eric Houle.
“We’re doing some things here that I know would raise eyebrows,” said Houle. “On top of increasing mileage, there’s other things we’re doing. Cam’s promised me he’s not going to talk about the stuff. Why, all of a sudden, can Cam, in the last lap of a race, leave these all-Americans behind as if they’re standing still? I mean, it’s more than just distance training. It’s going outside the box in a lot of aspects of training. I truly understand this may not work with another athlete again. But I know this: It worked with Cam.”
http://www.thestar.com/sports/london2012/athletics/article/1219026--london-olympics-cam-levins-racking-up-huge-mileage-and-big-results