Show prep is vitally important when you're solo. You have no one to bail you out.
With another co-host, show prep is much easier and takes much less time. My prep consisted of about 15-30 minutes with my co-host about what we were going to open with and current events we wanted to touch on. We would research our own talking points to bolster our views. Improv came naturally. We were both avid sports fans, well versed on a lot of sports and we could talk about anything. Having a guest or two per show certainly helped, and we always had a segment where we let the advertiser come on -- all of our shows were done via live remote.
We weren't Mike and Mike, but we were pros, and our show came off as such.