It would be helpful to first consider exactly when you are planning to visit Maine and the Acadia National Park. If you have not planned your trip yet, perhaps that will be an important deciding factor in helping you determine which months you should visit during. For instance, it will likely be far colder closer to the end of the year in the months of late November and December than it will be in September and October. However, just because you plan your visit in September or October certainly does not guarantee that you bypass cold temperatures and the potential for snow or ice! Weather in the Northeast can be unpredictable during the late fall.
The bottom line, no matter when you plan your visit, is to pack plenty of layers. It is a far safer bet to over pack and have to take off layers, than to be stuck out on a run feeling too cold and not able to layer up. If you plan to visit a bit earlier, you would probably be find to pack a long sleeve shirt, long tights, and perhaps a vest. A vest is definitely a wise choice for colder temperatures because, while you pump your arms and legs while you run (which helps them to warm up quicker), your core does not move around as much, and as a result, does not create as much heat. A vest will ensure you keep your internal body temperature higher, which in turn helps you to be warmer. If you plan to do your runs in the mornings or evenings, it would not be a bad idea to pack running gloves and a hat or ear covering as well. (And know yourself, too. If you know your feet have poor circulation and tend to stay cold, then definitely pack multiple socks.)
If you plan your trip to be in the winter months, then you need to ensure you have several layers on. That means several pairs of socks, a base layer up top and on bottom followed by a long sleeved heat shirt (and maybe even a third layer that is a jacket that can be easily zipped off and tied around your waist), a pair (or two) of long tights, and definitely something covering your head and ears, which are infamous for getting too cold too quickly and being susceptible to frostbite. You also need to think about your shoes. If there is snow and ice on the ground, your minimalist road shoes might not be the best option. You'd be better off with a thicker trail shoe designed to help insulate your feet.