Polly,

The first time I did this ride I decided in January to do it and it was only my third organized ride. I was in shape from running 4x a week but was a nervous wreck thinking about a ride of this length. And BTW running fitness does not mean you are in shape for long distance cycling.

What did I do? Rode, rode and even when I thought I might have rode enough I rode more. You have 6 months to train, you will be able to be ready. Look for organized rides that are designated as training rides. Maybe organize or find a local group. If you like find a century training program, I am not into anything that structured myself. I only did one organized 76 miler after signing up, all other rides were on my own. I would do short 12-15 milers after work and long rides on the weekend. I averaged about 100 miles a week and still ran 3x a week. I was still nervous but prepared. The first time was a great time (as was the second year using same method). Others on my team who had rode just on weekends and shorter rides on the weekends felt miserable. IMO getting time in the saddle is the #1 thing.