If you do them properly (i.e. with good rests between reps), the strides are essentially alactic. Their primary benefit, then, is on your neuromuscular system, promoting economy and fluidity at higher speeds.
My college coach called them "pickups," which I think is a more helpful term*: you jog a few steps into the rep, then gradually/smoothly pick up your tempo--the frequency of your steps--to accelerate. You'll work up to a fairly high speed, hold it for a few steps, and ease off. Get a good walking rest (some just walk back to the start--I especially recommend this on windy days, because you generally want to have the wind at your back when you do these), then go again.
The total length for a given pickup will probably vary between 60m and 100m. As you become more skilled at them, within each rep you'll probably build to a somewhat higher peak speed--and keep it a few steps longer--while still maintaining complete relaxation and control; hence the distance of each rep will probably increase.
As with most physical skills, pickups *initially* are best learned when relatively fresh--fairly early (though after some warmup) in a training session, in other words. But once they're well-learned, they can be done in the more traditional place, following a distance run. (But try this sometime: get a bit of warmup, do some pickups, and *then* go for your run. You may be surprised by how well it goes.
*I do realize that most people call them "strides," but sometimes that term confuses people and makes them think that lengthening one's stride is the goal of the exercise. It is not (although your stride will probably lengthen a bit, without any effort, as your tempo increases): the idea is to develop/maintain smoothness and control.
That's why just blasting off for 60-80m sprints is not the ideal way for most people to do strides--although even doing them as sprints is better than not doing them at all!