Having not cycled for a few years I decided to buy a 'sturdy' bike to use for day to day purposes - then, I was inspired by a story of someone doing LEJOG. I thought that I would have a go!
Having kept myself reasonably fit over the years (I've done 5 marathons, the last in 1991) I then had a period of leisure where I put on too much weight and decided to do something about it. So the gym it was and still is a regular part of my weekly routine.
No matter how much you do at the gym, it is no substitute for getting on your bike and breaking in the parts that need to be broken in. I cannot emphasise the need to do this sooner rather than later! Also, invest in a good saddle, I use a Brooks B17 Standard - what a fantastic saddle it was and still is, but it did need a few hundred miles to break it in. Invest in a cover for it too.
I also invested in some SPD pedals.
I did several rides in the build up, notably The London to Brighton Bike Ride, which was 54 miles and to my friend's in Hythe, 68 miles.