If you're a distance person, focus on increasing the tempo, rather than the stride, as you speed up on these.
I think that's why my college's coach called them "pickups," as in pick up the tempo, rather than "strides"--which some people interprest as a deliberate lengthening of the stride.
(As you get toward top speed, your steps probably *will* lengthen a bit, but it shouldn't be anything that you're consciously making happen. Think turnover--frequency of steps--instead. Same thing if you run strides on a very slight downhill: focus on increasing tempo and keep the feet under/behind you, rather than reaching forward with the foot.)