Well, I went out on my stretch of flattish 10 mile country lane and clocked 30 mins 10 seconds. (Yes 10 bloody seconds).
I lost some time stopping and turning around after 5 miles then a slow RLJ at a tiny hamlet and temporary lights so I know I could have got (just)under 30 minutes. I'm pretty happy with my 20mph average though.
Unfortunately my personal rules state that I can't bleat about any obstructions (traffic, road works, cats, ducks, and so on) so I need to get out there again and try again, I'll be happy with 29 mins 59secs.
Also I did it on my touring bike and rode 8 miles to the start, next time I'll use the Orbit Road Bike.
I can still remember the pain so I'll leave it a week. Chapeau to all you 24 minute types.