There's two things you need to worry about:

1. Fit/comfort on the bike
2. Fitness

The first one is very important when biking long distances. You need to get a professional bike fit and you'll need to work on your core, flexibility and get really comfortable shorts, shoes, socks and some chamois "butter" (DZ Nuts is good, Bag Balm and there's others). This is actually more important than fitness. If you get a sore back, saddle sores or Shermer's neck after 5-10 days, your ride is finished no matter how good your fitness is.