That's tough, you already mentioned the biggies in the direct area. If you're willing to range a bit further, there's the Virginia Marathon and the Two Rivers Marathon (both March 22nd, no experience with either race personally.)
Also a bit further, in October is the Atlantic City Marathon. I haven't done this race, but it's the same race organizers for the April Fools Half in the spring and they've always been VERY well run. (They also have the option of doing bib pickup the weekend before at Dicks Sporting Goods in Cherry Hill, so you don't need to trek out to Atlantic City twice. I do that, and then drive over the morning of the race, I've never stayed overnight in AC.) Fast, flat, and 8 miles of boardwalk and sea breezes.