I weight about 155 (pounds) and have about 6-8% body fat. I have plenty of muscle, however I kind of want to bulk down to become more efficient at running. What can I do with my diet in terms of things to eat and avoid that will help me drop a bit of muscle mass?
EAT PROTEIN. DO XFIT. GET SWOLE.
Don't do it.
At that body fat level, you're already at the low end of your natural weight. If you turn into a sub-150 pound twerp you might get better at 10k, but not only will you be sacrificing all the rest of your fitness, you'll also lose your speed over shorter distances.