I ran competitively back when Americans used to win the big marathons here in the US. Dang ole Kenyans are machines.

Go to runnersworld.com and dig. There's more good info there than you'll ever use.
The short answer is to follow a staggered training routine, incorporating progressively longer runs with speed training. You should already be doing fifty mile weeks, with one day off for rest.
If you'll describe your current routine, I can give you some pointers. PM is fine if you want.
Dan
P.S. Here's something that worked well for me. On a short to medium distance run(8 miles), and after at least a mile to get good and warmed up, pick a spot about a hundred yards away and slowly accelerate to maximum speed, taking care to keep your stride from shortening. Ignore the lactic acid building up and hold as much as you can until you reach the spot you picked out. When you reach the spot, ease back down to your regular pace, but do not go any slower. After you recover, do it again. Keep it up until you finish the run. Long intervals like these will build both quickness and stamina as well as improve your ability to deal with hitting the lactic wall.