Not that old yet, in my 40's, but I'm interested in this topic because I want to retain what little speed I have.
What science says is that strength, flexibility and elasticity decrease with age, and these all contribute to stride length and speed. Running speed depends a lot on the contact time of the footstrike, and this in turn requires specific strength. As we age the contact time typically increases, so in order to keep the contact time from increasing we need to train for the necessary strength.
I agree with everyone who does quick 60m hill repeats, these are truly great IME. Besides that you can do various basic strength exercices like lunges, squats, or even the one-legged squat (pistol), but don't forget the rest of the body either (ankles, core etc.). Once basic strength is in place jumps, bounding and plyometrics can help. A little less extreme but maybe even more usefull are certain drills like in this video:
http://blip.tv/running-times/mastering-technique-drills-2599505
I'd say the main thing is that to have an efficient stride, regardless of cadence or length, you need to have speed and strength in reserve. Something I found out recently is that training for speed seems to require a complete spectrum of exercises from low speed with maximum resistance to low resistance at maximum speed. The first is taken care of by low rep (3-6) weight training, the second by jumps and other explosive movements. Hill sprints I guess would be somewhere in-between. Now I'm going on about speed, and you might ask is that really related to stride length? My answer is that to increase speed either cadence or stride length or both must increase. Increasing stride length requires increases in force generation and increases in cadence require faster muscle contraction. In both cases we are dealing with some combination of speed and strength, which are in turn the focus of the training examples I suggested.