Weird timing, but I was reading the Book "Run Strong" by Kevin Beck, and Chapter 2 (written by Greg McMillan) has a section called "Stride workouts". Here is what is says:
"Stride workouts are a great way to introduce leg-speed training into your program. In fact I call the fast portions of leg-speed training "strides" instead of sprints, as the word "sprint" conjures bad memories for many runners or makes them think that they must run at an all-out speed, which can lead to straining and poor form.
In a stride workout, warm-up by jogging slowly for 10 to 30 minutes, then begin to alternate 15- to 30-second strides with 45- to 60-second jogs.
A great advantage of this workout is that it does not require a track and can be done on trails, grass, or roads. I start runners with 10 or 15 strides and gradually increase the number to 15 to 25. Remember to jog slowly between each stride to allow full recovery. Stride workouts are especially great for people without access to a track or who are so time-obsessed that they'll start racing themselves if they only do straights and curves.
You can also do stride workouts at the end of an easy run; it's your choice. My suggestion is to do them in the middle of your runs, so that your body and mind are not tired and you can maintain good form. Sometimes runners get sloppy with strides at the end of a run."