I've ridden two fixed centuries now. The route I've taken has been the same both ways--the gateway trail out of MPLS/St. Paul to some state park down a highway to Stillwater and back, then riding part of the Grand Rounds in the TC to make it 100 even; if you really want to know elevation changes holler and I'll google it for you, but I can tell you that the gateway is pretty flat (and thats probably 25-30 miles), the highway into Stillwater is rolling hills, and the hills out of Stillwater (we take a different highway out to meet the gateway) aren't terribly long but require some mashing. I didn't bother to change my gearing at all. On my first ride I set out fixed as a sort of personal challenge to see if I could do it, figuring I'd flip to my freewheel if I couldn't handle it. The rabbit gear calculator has me at 70.3 gi, 48/18. That gets you to 16.7mph at 80 rpms (and I was surprised to apparently be doing that on the way into Stillwater, so sayeth the speed limit, "you're going this fast" sign), and up to 20.9 at 100. I don't know that you'll be maintaining 100 rpms for any extended period, but I'm pretty ill-experienced with longer rides, and maybe your fitness level blows mine out of the water. If you're doing fw/fw on a flip flop hub, why not start there and drop down to 60-65 gi for the hilly rides if they're really bad? Or adjust up if you're substantially more fit than me and know what kind of mph you push based on a given gear ratio?
P.S. This was riding with a friend who rides substantially less than me, so a pretty lackadaisical pace. I pulled ahead of him at times for funzies, but ultimately wanted to hang back with him. If you're going more uptempo with your friends, you can definitely go higher than that.