You definitely want to work up to sprinting, but it doesn’t have to be a long, drawn out process if you’re only 21 and were athletic in high school. Like it was said over and over, you don’t need to run at 100% intensity. Mechanics are also very important. But, in all reality, you probably only need to focus on getting your knees up, landing on the balls of your feet and landing with your feet almost underneath you rather than out in front of you.
This kind of over-striding is a very common reason for hamstring pulls, and the advice above about running uphill is spot on.
Here’s a really simple way to ease into it and get yourself into shape quickly. Do Tabata sets on a treadmill. A Tabata set is basically 20 seconds on, 10 seconds off, for 8 reps. So, you’ll run for 20 seconds, then hop off to the sides of the belt for a 10 second rest. Just make sure to hold onto the handrails when you get on and off and practice this until you’re comfortable.
I suggest you warm up thoroughly before starting this. When you’re done with the 8 sets, rest for 4-5 minutes and repeat it one more time.
Start out at 2% incline and 6 MPH. When you can complete the whole thing, move the speed up to 6.5. When you can complete it all, move up to 7.0. Then you can start increasing the incline by 1%. There is no reason to increase by more than .5 MPH or 1% incline.
This will basically turn into a “sprinting” workout but in a safe, progressive way. Good luck.