too much trans fluid

i changed the trans fluid and filter on my 99 durnago; problem is, like an idiot, i jsut started pouring new fluid in to meet its capacity. now it's got too much in it, and there's a puddle underneath after driving it for 20+ minutes. have i blown seals, or is there an overflow valve or something? if so, how tough is it going to be to change seals myself?