You are confusing two things here. An interference proceeding is used if two parties are trying to get the patent, both claiming to be the inventor. If you are being sued for infringement for something you have been doing all along, your defense is not "interference", it is "invalidity". A patent is only valid if it is, among other things, novel. If you were doing this or using this before the filing date of the patent, and can prove it, they're boned.