I know, I know...it's a funny word. Stifle the giggles and listen up for a second. I'm about to explain another way to challenge your running game: According to the dictionary, "fartlek" is a Swedish word that means "speed play." And that's exactly what fartlek training is. Intense bouts of fast-paced running sliced into longer, slower runs.
There are many ways to implement fartlek training. The easiest of which is to give yourself a decent warm-up, then haul ass until you can't, at which point you'd bring it back down to your normal pace until you feel up for another session of hauling ass. Repeat as often, or as little, as you'd like. You can also map out your fartlek training by predetermining certain landmarks along your way that will be starting points for speed play. This helps you anticipate the additional effort, and can make it easier to recover in between.
Not only does fartlek training help increase your speed, it calls on more muscle fibers. Fast-twitch muscle fibers, to be specific, which we use when our muscles must act quickly. On the other hand, slow-twitch fibers are built for endurance. So when you use both in a longer run, it challenges each and every aspect of your muscle to build a stronger, fitter muscle in the end.