I am in awe of some of the distances on here! 
Ok, so I'm reasonably fit, I run 3 times a week, do pump 2-3 times a week and I cycle everywhere. Over winter when the weather was rubbish I did spin classes instead. However, I have never raced, and I usually ride a hybrid with nice sit up and beg handlebars and a pretty bell! 
I'm going to have to hire a road bike for the race which is 4 flat laps, 21.2km. I have been angling for a road bike for years, but dh doesn't really understand why I possibly need a road bike as well as my hybrid and mountain bikes. I like a bit of offroading from time to time - live close to swinley forest which has some fantastic singletrack. I am worried about the transition to a road bike. I was wondering about dropping my handlebars for now to try to get used to a lower profile. I do tend to tuck my elbows in and get low when in a hurry anyway. 
I went out for my first attempt yesterday, on my hybrid and managed 8.5 miles in 30 minutes, before I hit Maidenhead and with the roundabout and lights dropped to 12 mph for a further 3.5 miles. I need to find some more country roads. Do you think going back to Spin classes will help with training, or are they a waste of time?
Also should I stop running for now? I've cut back and will replace my sunday long run with a long ride instead, but I will stop if need be.
also any idea of a training plan, how often I should go out, and any tips at all would really be appreciated. I normally do at about 2 hours exercise a day, I'm wondering if I will have to cut back a bit to focus on the cycling.