No worries. My generally sweet 3 year old sometimes chooses to go the uncooperative route, always while in public, and I'm pretty sure that my own reaction is not as calm as it could be because its like I feel i've got a whole spotlight on me! Also, you could be feeling like the situation was worse than it was for this reason.
Other ideas for dealing 1) give 5-minute, 2-minute, 1-minute warnings that you are leaving. Eases the transition time. We even sing the goofy song "2 more minutes to XX time, 2 more minutes to XX time...." it helps so they are not caught off guard when you say its time to switch gears. If there is fuss, you just say "you've had your warning."
2) Make sure you have extremely firm and consistent parking lot rules. I usually say, "THIS IS A PARKING LOT, WE MUST HOLD HANDS." If my son chooses to be stubborn on this, and after giving chances to improve. I will just firmly scoop him up by any means necessary. I argue that when trying to keep your 2 kids safe, you could find a way to hold both at the same time. A good teaching moment for later for your son, "see how uncomfortable that was for you and your sister? We have to have you hold hands in the future...." I just don't play around when it comes to things that are non-negotiable. Like tooth brushing. I hold my son's head so it is immobile and get in there and get it done (after he has "his turn" I need "my turn" so it gets accomplished). What am I going to do - let my child go without having his teeth properly brushed? No - I don't find health or saftey issues things that you can let slide. Your not abusive, you are being a parent who doesn't tiptoe around issues of health and saftey.
3) If these sort of tactics do not work easily with your son, then I suggest giving him challenges like "lets see if we can march our feet over to the car" or "i need your help to find the car" etc. these sort of things work wonders. Diverts the kids attention from what they are being, in essence, forced to do to making something fun (for them)
4) While I've always been in the Chicago area, I know how you feel about being at a new preschool. For the first several weeks I felt like such an outsider even though I lived here for over 10 years! Keep putting yourself out there and meeting the parents. I'm sure we're not that different from the Ohio ones ;) As my husband said, "you need to find like one good mom friend at the preschool who can help you get better connected to the others" I'm still introducing myself and we're almost 2 months in...but its working and now I like these parents a whole lot!!