My personal goal this season is to average 15+ mile (trail/easy xc) days. Now, this is for trips lasting only for 3-4 days, and with a near SUL rig to boot (13 lbs start, 8.5 finish), so it's not really a marathon, but I've gotten progressively more serious about training.
I really enjoy walking, but the distance/time required to develop this level of conditioning was taking too long, so I bit the bullet and started running (again.) Now, I've always been a big critic of running, but the recent trend in barefoot/flat heel running perked up my interest.
(I don't like running because in my mind, our paleo ancestors weren't jogging around tracks or on hard streets/sidewalks. Rather, they were either walking to get from place A to place B, or they were jogging/trotting while pursuing injured game.)
Not to belabor a well debated topic, but I think it makes sense to walk @ a really fast pace and/or jog/trot @ a really slow pace **over non-man made surface areas**. And because running has a 4x caloric burn over walking, I can compress the time by 75%. (Running for 1 hr burns 800 calories, while walking burns 200. If you move to trails and/or packing, the differential remains.)
I bought a pair of NB MT 110s back in January, and after walking in them for a few months, started jogging in them along some of our trails. Right now, I'm up to 5-6 miles/day, split into two sessions (2-3 miles per session). My next goal is to combine them - basically, a 10k each day.
I figure if I can reliably run 6+ miles/day, then I can reach 15 miles/day without killing myself. Even if the net energy/endurance requirement is increased on the order of 50%, I think I'll have enough reserves to motor the distance I need before heading out.