It's not ruined. You just shot twice as many frames as you need. Do you want a 30p timeline? Does it matter to you? Is the slo mo footage supposed to mix in the same program as your interview? If so, then you'll want to great your program (timeline) as a 30p timeline. then conform your slo mo footage to 30p. And convert your interview footage to 30p either using a separate program (compressor, mpeg streamclip, etc.) or just drop the 60p interview footage into the 30p timeline and let the program convert it yourself.
(Real time performance may be deminished if you choose the 2nd option because you're computer is having to calculate constantly while editing.)