I did this this year, 2:24.xx something, and it was bloody tough, it climbs after 1 mile into each lap, more a tough undulation, then at the 4 mile point, and bleedin big hill that knackeres you...
...the two last laps are shorter versions and miss this massive hill, but it is very undulating without this...
... personally, I reckon its around 3-4 minutes slower than a flat 20 miler...
I'm doing Trimpell 20 instead this year as it's a week earlier, so will give my legs longer to recover ;-)
Pug