I coached my brother's soccer team when I was in HS, I ran them to death every once in a while. They got quite fast, but I also made sure they did technical drills, such as dribbling in a small square moving around, and also small sided games.
In order to keep them interested, you have to mix it up. Same concept, but in a different manner. I also recommend a few warmup laps, stretches, then a bit of running. Thats how I would open and close practice and in between, drills.
You're on the right track though. The hardest part for me was keeping them into it.