If you have a starter made, that will help alot (or pitch two vials of liquid yeast). The fermentation shouldn't take longer, but it might take longer to age it. Also, you might want to consider dryhopping even if you didn't plan to originally. Your beer will be out of balance if you didn't add more bittering hops to counter the additional fermentables and you might be able to cover that a bit with some dry hopping. Make sure you oxygenate before pitching the yeast to keep the yeast healthy and happy and from being stressed.
I can't think of anything else right now!
