Ok tried my road route on the MTB (still with wide tyres and lots of mud on it), giving similar effort to my usual training ride (usually consistent).
14.4 miles, Average speed was 1.9mph slower than the road bike, 7 minutes longer and top speed on the downhill reduced by about 5mph which mathmatically suggests around 40 minutes longer over 80 miles but I think that gap will increase significantly as it felt a lot more effort to keep moving.
Problems noticed mainly was the riding position made it difficult to get down low to avoid wind. Pro - A lot more stable and more braking power.
My Road Bike is a Specialized Allez - not sure about the strength off road so may still change my tyres on MTB to go as narrow and free rolling as poss, may change the bars or add some clip on tri bars, I expect that would increase the average speed by over 1 mph to bridge the gap and give a good cross alternative.
I have bravely entered the one day event, sucker for punishment. I have a very light hartail mountain bike and a road bike. Thought I'd go on the MTB fitted with 1.8" tyres instead of its 2.3" mud cluggers. Should reduce resistence significantly and a cross type tred. Any recommendations out there?