Sounds great, good luck!
I was a discussant a few years ago on a subject only vaguely related to my research.
Here's how I prepared for that:
Firstly, I took a good guess at who'd be in the audience and who'd be likely to ask questions (i.e. who are the eminent people that are going to be there). I did some research on those people (areas of research, political perspectives, big theoretical contributions etc.) so I was prepared for who might be asking me questions and what agenda they might have.
Secondly, I spent a good few days acquainting myself with the "core" research of that field so that I could talk in very general terms about, for example, particular theoretical changes that'd happened.
Thirdly, when I was introduced I made sure the host was clear about my research area so the audience knew I was only vaguely related to the topic of the talk.
If I were you I would get in touch with the conference organiser and ask if they have any particular ideas of how they see the discussion being structured. For example, do they want you to be the critical voice? Do they want you to be an advocate? Do they want your summary with two or three points for discussion?
If not, then I would just be willing and open to have a frank discussion. So, be honest about the good points of the talk, the bad points, the points you'd like to discuss more, the points you think there could be more research on etc.
I think there's two ways to approach these kind of events.
i) from the perspective "what would a senior man do?" - just bullshit and bluster through, bringing everything back to your research and own agenda.
ii) or just be yourself. Have open and frank discussion but be honest when you don't understand something and pass the buck (by saying something like "I'm not the best person to answer this point, perhaps we could pass over to one of the other discussants whose work is more closely related"
I took the second approach when I was a discussant and it worked really well for me. I don't think I came across as forthright and as much like a "future leader" as I'd have wanted but I felt much more comfortable and actually started some very good collaborations on the back of it.