thanks for the pat on the back. i'm in the USA, so in my parlance a 'century' is 100 miles and a 'metric century' is 100k/~62 miles. 'back-to-back' is one century, two days in a row...not 200mi in a single ride!
100 miles on consecutive days is impressive, flat or not - the lure of food every hour, attractive riding companions and free beer must help a bit though.
Sadly I have only managed 2 hours at most as it is undulating around here to say the least plus I get bounced around on the dodgy road surfaces, some of which are only as smooth as farm tracks (and even with 32c tyres at modest pressure it does get a bit uncomfortable after a while). Still a good session for the legs and makes a nice alternative to a geared bike.
A friend of mine rode this http://www.cyclosportives.co.uk/epicRoute.htm
fixed last year and got round in Gold standard time. As you can see from the route profile there is precious little flat and how on earth he got down let alone up the 20% and 25% sections I have no idea. Almost all on narrow country lanes with poor to middling surfaces, an awesome achievement especially as he could still walk at the end.