"I hate to tell you but I have to run just now, but I'd love to catch up with you again later and really talk about this, can we get together sometime soon?"
"Sorry Robert, I have to get going, but can we meet up again soon? Is it alright to call you/e-mail/see you here next week about the same time?"
"I have to let you go just now but I've really enjoyed this chat, can we speak more next time? I really want to hear more about this."
"Thank you so much for this chat -- I have to go right about now, I'm so sorry! But, I want to hear more about this, can I take a raincheck and catch you soon?"
Always include:
1) An acknowledgment that you have really enjoyed this talk.
2) Information that you have to rush away just now.
3) But an acknowledgement that you want more of this/another talk, soon, and try to "book" that happening even if only loosely, like mentioning that you want that future talk to hear more.