Here's some perspective from a competitive cyclist...
A world class runner would, generally, fare very well after a few years of training as a cyclist. They would get shelled in anaerobic events like criteriums because they have no top end, but they would naturally excel at hilly road races.
Triathlete Chris Lieto started taking up cycling in the hardest district in the nation, Northern California. He competed in the Pro 1/2s and recently won a difficult stage race. These are domestic pros he raced against here.
Three years ago, collegiate runner, Justin England, decided to take up competitive cycling. Just like everyone else, he started in the amateur newbie class (Cat5), where he won the majority of his races. He cat'd up in quick fashion and became a pro. Not just any pro. He became Chris Horner's right hand man at Webcor. He also recently smashed a 15 year old hill climb record at Mt. Tamalpasi by 3 minutes!!! This was a record that pros had been trying to beat this whole time because it comes with a cash prize.
In the same sense, I do not hesitate to say that the majority of world-class runners could enter the roadie world and become domestic pros within 3 years given the right training. The folks that "dance on the pedals" in the mountains (just like Lance), are usually runners as well. The sports lend themselves well to each other.
Finally, it's not like Lance has zero running background. A 2:30 is more then manageable for him after a year of real training. And yes, he's "retired", but he retired at the absolute pinnacle of his sport and only a week ago. It's not like he's out of shape now. Quite to the contrary, many folks think he's in the best shape of his whole career.