I'm not a huge fan of Potomac River Running. One would think that the only quality running store in Sterling/Ashburn/Leesburg would have a better selection, but it's mostly just clothes and shoes. Not a lot of accessories or bars/gels.
The staff isn't very friendly, either. If you don't look like a runner (long, lean build), they practically ignore you. They're definitely running snobs (which I understand, but I've run 5 marathons!) and they need to work on their attitudes. I prefer to give my business to another running store that has much better selection and much friendlier staff, even though it's further away.