I run the keswick half marathon on a regular(ish) basis... though I don't do a lot of training for it.

Cardio shouldn't be an issue (sounds stupid but bare with me )because not excerting yourself a lot... just for a long time. In my experience I've never been gassed out... I just can't bend my legs after about 9 miles! :P I've got the lungs for it just from doing boxing conditioning but not the muscular endurance in my legs.

So you want to be getting miles in them... run for distance, do a lot of walking and try to complete a few half marathons as part of your training. I'm guessing you should be aiming to do 5-8 miles a few times a week.

From the little I know about long distance running, I know that they try to clock up a certain amount of miles a week rather than just per run.

My dad was a pretty good cross country runner, to bad he's away for the week or I'd ask him! :P

An area I should probably be more knowledgeable on... I just haul myself on long runs with my boxing club/mates when we go on holiday etc... never train to improve at them