I've decided that my current bike perhaps is not the best bike to be using to train for the challenge. It weighs in at over 15kg, ie over 33lbs. It's close to a quarter of my body weight! Going up hills pulling an extra 25% weight is really hard work, and I get very tired very quickly.
I bought this bike a few years ago when I lived near the New Forest. Perfect at the time for short rides with the family in the forest, not so good out on the road trying to get my fitness level up.
I've therefore gone and spent my money on a new bike from Wiggle. It's a Mekk Poggio 2G P2.5 Ultegra. It looks awesome on the pictures, weighs about 8.5kg (19lbs) and should arrive in a few days time.
Looking forward to see how this changes things