I claim no expertise in this area, although I am riding faster than I used.
When I resumed cycling, in March 2006, my normal 16-mile loop would take me more than 1h 30m. Gradually, fitness improved and the time came down.
My first 100k audax got me in bang on the wire, in a state of total exhaustion. That was almost 2 years ago. Since then I have completed a few 200s and had trouble with the time, partly because of faffage/eating. My best I completed with about 30 minutes to spare, and I wasted time with a puncture.
My last couple of 100ks have been completed with an hour or more to spare. I find that my Mercian is quicker than the Thorn, but the Thorn's fatter tyres give me more confidence.
On the Mercian, I once completed my normal 16 mile loop in 58 minutes, but I was breathing very hard and covered in sweat at the end of it.
When I did the Dun Run, which was only a smidgin short of 200k, I did it in about 11 hours total.
I suppose what I am saying is that you should aim to knock time off your basic ride and do longer ones and try to keep the time down.
I'm doing a hilly 200k in a couple of weeks. I'll probably take the Mercian on that.