I use extra run off for my starters all the time. But that's usually for a batch down the road. Ideally you want to make your starter a few days before brewing to give it chance to actually grow yeast. That said, you can make a starter, pitch the yeast the day before, or even the morning of, brew day to just proof the yeast and get it active. While this isn't ideal it is far better than just pitching a vial (for ales, anyway. You really need to grow yeast up or pitch several vials for a lager).
That said, if you plan to proof and pitch active be sure to have a packet or two of dry yeast on hand in case the starter doesn't get started.