I just finished my first marathon and also had difficulty getting increasing my stamina so that I could complete the necessary long runs. I trained 3-4 times a week using Jeff Galloway's run/walk plan. My goal was to finish - which I did.
Here's what worked for me:
Rest the day before a long run - usually I did no workout or just rode a stationary bike for 30 minutes easy and got a good stretch in.
Eat a high-carb meal the night before -plain pasta worked best for me. Get a good night's sleep the night before.
Get up early and eat a breakfast consisting of intermediate-glycemic carbs - you will need them at about the 1.5 hour point. Avoid too much fiber or milk products.
Drink some fluids prior to starting to run - don't overdrink - just enough to pee clear.
Make sure you are taking in enough carbs during your run - Gu and Accelerade worked best for me. Whatever you like and can tolerate is best.
Take enough water/fluid with you or stash it along your route the day before.
Start early enough to avoid the heat. Trying to run farther than you ever have before is extremely difficult if you're also battling the heat. Don't make it harder than it already is.
Start slow. If you go out too fast you'll burn up all your glycogen stores early and have nothing left for the end of the run.
Give yourself plenty of recovery during the week after a long run - I only ran 3-4 times a week and once I got up to 12 miles only increased my long-run mileage every three weeks. I did 8-10 miles during the "easy" weeks.
The key for me was training my body to get past the 2 hour point. Once I did that, it was easier every time. I also did long combined workouts during the 'down' weeks. This consisted of a 1.5 to 2 hour total workout - 30 minute easy bike, 30 minute circuit training, followed by 30 easy minute run. I tried to do these workouts without supplemental carbs or not as much as when I ran. I think they really helped me increase my endurance without the pounding. But, make no mistake, there's no getting better at running without the running.
Increase the time of your short/medium runs during the week. Adding in a 1 hour run per week during the non-long weeks helped me a lot. But don't increase the distance of these runs the same week as your long run. The long run is key.
Finally - although it won't increase you're stamina, it will help your recovery. I took an ice bath after every long run - filled the bathtub with some ice and cold water and got in. Yes, it's as difficult as it sounds but my knees and other joints were very happy the next day. Much less soreness.
Post a reply on the Bulletin Board