Hi Lauren, Congratulations on your NYC marathon. A few from our running group ran in it also. Nike makes plus size running gear. I was at Academy and they now carry Nike plus size gear. C9 by Champion also makes some quality running gear. They have different levels of quality. Premium are the only ones I use. You can find it at Target.
I don't know where you live and how cold it gets but I wear Nathan breakaway sleeves. DriFit also makes warm running socks that really make a difference.