To improve your endurance, you need a plan and here's one that will challenge every level. Funny name but awesome training....
Yep, it sounds just like it's spelled. It's Swedish word that means "speed-play" and it's a great tool for training speed and endurance at the same time. We do a lot of this in our conditioning training, but for the workouts on your own, you'll be choosing the pace and the distance for each pace. If you go with a group of soccer friends, you can trade off "leaders" every 5 minutes.
An example of a good Fartlek workout would be:
2-3mile total run
5 minutes of warmup at gear 2-3
For the remainder of the run your default gear will be gear 3. The leader will then pick a landmark (a driveway, mailbox, palm tree, street sign, etc), choose a speed (gears 2 through 5), and the group will run to that landmark at that speed. The group will return to a gear 3 speed until the leader chooses another landmark.
When you are finished with your "speed-play", be sure to jog easy (gear 2-3) for 10-15 minutes and then get some good stretches in!
Have fun and I'll see you on the field!