You are looking for a pedometer. It will clip to the side of your pants or shorts. Initially you will need to figure out your stride length; for instance running 10 steps, measure, divide by 10 for the average. You could even repeat this several times and average the averages. Once the pedometer is programmed for you, it will count the number of times it shakes with from the impact of you running and let you know, even down to the hundredth of a mile, how far you've gone.
I got mine at RadioShack for around 10 or 15 bucks, and I know WalMart has them as well.
RS :
Walmart: