Bring the exact same gear/equipment (shorts, shoes, socks, pedals, etc) you use the most in your regular rides. Bring your regular hydration/nutrition for a 1- 1.5 hour ride (idea is to replicate indoors as close as possible your regular outdoor ride). Bring a towel and some music (it'll get boring). If done properly, the fitter will want to see how is the decay in your form as you get tired, so it'll take some time (hence the music). A change of clothes (skip if riding back home). I'd bring a six pack to the fitter (he'd end up sharing it with the rest of the staff).
Riding to the fitting shouldn't be bad, depending on form, the ride back after the fitting may suck, but being only 6 miles i shouldn't worry about it.

Good luck