Strap on a weighted vest and do interval sprints on a track or football field. Use only five to 10 pounds, any more will place too much stress on your lower back and knees.
Need another option?
Go old school. Take an old tire, put on a belt, and attach it to the tire with a 10 to 15-yard cord. You're having to work so much harder than simply running alone, and it'll help increase your stride. You could also run with a parachute - or even in the wind - for added resistance.