It would still only be -10 to your initiative. It's important to realize, as Xenon described, that using Full defense does not cost you your action phase in this or the next IP like it did in 4th edition (IIRC). It only costs your initiative score, which indirectly might mean you have no more actions in this round, but maybe not.
So when you move, and take a simple action, you still have a simple action left which you don't have to use, but when your turn is done, you can then take full defense (but like Xenon said, it's silly to do this interrupt before you're attacked). The interrupt action will cost you 10 and then the end of the IP will cost everyone 10 and thus begin the next IP.