First off, give yourself some credit! Running a marathon is no small feat, and running five marathons is certainly something to tip your hat to! Whether you can jog or run the whole way, or need to stop and walk a bit in order to finish, if you crossed the finish line of a marathon, you still covered 26.2 miles on your feet. That in itself says your stamina is already pretty good!
But we understand the desire to get even better. After all, it truly is a monumental moment and feeling when you can run an entire marathon by actually running. What an accomplishment! And building up your stamina to get to that place is not actually as hard as you might think - it simply takes a well designed training plan and the self discipline to actually stick to it. First, change up your workouts. Do not do all speed work or all distance work. Most people, when training for a marathon, focus entirely on the distance. So week in and week out leading up to the race, they are logging a lot of miles. Which is good, as endurance is needed! But incorporating speed work will ensure you get your heart into great cardiovascular shape, which translates to increased stamina.
And to make sure you do not get bored, switch up your speed interval training. A few times a week (two to three days), do a different speed workout. Some examples would be a pyramid style work out, in which you increasingly get faster and faster for shorter spurts and then gradually work your way back down. If you can't get to a track, the treadmill is an excellent option for speed work. And aim to work in some strength training as well. Lifting heavy weights will work wonders for your heart, which again, leads to rewards in the way of increased stamina.
It is also a good idea to give your running muscles and running brain a break. Try cross training with other forms of cardio. Hop on the bike to switch it up and alternate easily between distance and high intesnity intervals, or throw on your goggles and swim gear and hit the pool for some laps. Changing up the type of cardio you are doing means you changes up the motions your body is used to performing, tricking the brain and shocking the muscles and leading to increased cardio performance. But don't skip on running! Slowly increase your mileage by 10 percent each week, if you are trying to figure out to up your distance to get into proper marathon shape.