We definitely won't be out for longer than two hours, but if you're worried about lasting that long I would advise bringing food (raisins, clif bar, gummy worms, whatever) and as much water as you think you will need. If we do Merrifield it will be around 25 miles.
With the weather getting chillier and the wind being as unpredictable as it is, my suggestion is to get out and ride while you can. As long as the posted ride isn't a "training ride" or "hard ride," anyone is welcome to show up, even if they haven't ridden with us yet this year.