If you're wondering whether it's valid, there is actually a fair amount of of research on beets (or dietary nitrates) for endurance.
It's one of the few (5) supplements
accepted by the IOC as having good to strong evidence for performance enhancement. Even with the broad benefits seen, there are still people in every study who just don't respond at all to it. So, your mileage may vary.
If you're wondering the best way to use it, that is debatable. In general, with an otherwise balanced diet, you don't need to use a beet supplement regularly and will benefit more from training without it. Race week is when it comes into play, giving you a little boost. Some people just use it race morning, others dose a few days leading up to and including race day.
http://www.extramilenutrition.com