Pete, I would be holding off on these big rides for 6-12months until, you have strengthened your riding. No disrespect intended, but trying long rides before your body has become accustomed can give you strain injuries. Keep extending your rides, fine tuning your bike fit(position) until you're at 5+ hours without discomfort(pain).I've never done anything but would really like to. This kind of event - relatively entry level, seems reasonable. Was also looking at the Lincoln 262 - longer but supposedly very flat.
By the way, Lincolnshire isn't really flat, it is compared to Yorkshire, but there are the Wolds which have some steep climbs, Id be surprised if a big route like that didn't take in the Wolds too