If you've already got in, it's not an interview.
If you really don't think you'd like to talk to the profs for that long, either tell the person who setup your visit, or just cut some of the discussions short. I'm sure the profs have plenty of other stuff to do with their time so they won't mind at all.
But in all seriousness don't worry about not having enough to talk about. Just ask them, "Tell me about your research" and they'll blab on for an hour with no trouble at all. In fact, this was kind of a problem when I was visiting schools last year, so in general I recommend asking for the 5 minute summary first.