In my opinion, and take this for what it's worth because I'm a fellow newb, but the philosophy of "sprinting" is on the surface about running, but truly about sprinting from a cardiovascular and even a metabolic standpoint. In other words...run if you can, but in lieu of running, sprint by doing an activity that gets your heart racing, pumps blood, adequately (and safely) raises your BP, for a short amount of time. Also preferably getting your lower body muscles involved so as to include venous heart return from your lower extremities to move blood, and lymph back to the core of your body.
I think mark is suggesting to merely mock a cardio and metabolic sprint scenario, much like Grok might have needed to do when hunting, or faced with a fight or flight situation.
Examples of this:
My number one: ROWING. We row in crossfit and doing a burst of 300+ Meter "sprint" is brutal. Try it, you'll see. Be sure to use your entire body, taking the bar all the way to the machine, push with your legs (Leg involvement) and pull back to your chest in sync with your legs. You'll see, the movement, when done right, gets your entire body involved especially your core and legs. Until crossfit, I thought rowing was for old men looking for a low-impact workout....boy was I wrong. I'd rather deadlift 300 lbs any day. Best of all, on most machines you can adjust resistance...
If you can tolerate the movement, try Jump Roping. It's a sprint...trust me. Do double unders with a speed rope (2 spins at a time). Double unders will take some time to master, but in trying to learn to do "Double Unders" you will get a great work out. To do so, pick a number (20reps, 30reps, 50, etc) and do a set for time. If you mess up, just count the time it takes to stop, reset, and get started again as part of the over all time. And then when you repeat a work out, attempt to beat your old time (Mistakes and all). When you get better, increase the number of reps you need to do..and time that. Beating your old time encourages the sprint. You'll see that Jump roping is a full body workout as well. (That's why all fighters, boxers, mma do it!)
Here's a link to my blog with some speed ropes:
The Resolution: Speed Jump Ropes from "Again Faster" for Crossfit
I purchase mine from Again Faster. I'm sure there are plenty of sources, but you really want to get the wire type ropes. I REALLY love mine.
Hope this helps!