There's no arguing that if you're looking to improve your speed, doing intervals on the track will help. A standard 400 meter track makes it easy to run any distance in repetitive format. I prefer simple workouts like 400s at 5K or faster pace with a 200 or 400 meter jog in between.
But I don't like running on the track for various reasons. It is always challenging finding/making the time to get over to the track when it is open. Running in circles isn't particularly fun for me. And I tend to get injured on the track.
As an alternative, a few weeks ago I started to do a little stealh interval work on a local road. Starting out with only 4 repetitions from one point to another on a slightly downhill surface, I'll jog back to the start, a noticeable crack in the pavement. The finish is a sign adjacent to the road. Nothing fancy. I don't even know the exact distance (though I'd estimate it is about 450 meters).
I feel fitter already, having done this routine just once a week. I'm up to repetitions. This upcoming week will test my commitment with the Daylight Saving time change...one less hour of sleep this week!
So think about mixing it up in your own routine a bit. You'll definitely notice a difference.