When I first contemplated doing the ride I was 38 years old and of average fitness. The message here is you dont have to be the fastest or the strongest person, just determined and without any hindering injuries.
My training would have to be squeezed into a busy life schedule somewhere between work, four kids, my wife, her job and sleeping! I do like to run a little and used this to get a base level of cardiovascular fitness. Running was handy because in the winter, cycling is not as much fun. You could always work out in a Gym or swim, but I prefer to excercise outdoors. I would run a few times a week, maybe half an hour during lunchtimes at work and slightly longer on a weekend.
By the time the clocks went forward and British Summertime arrived I had a nice level of fitness. I had now purchased my bike and was happy to ride in the longer evenings. For the next 6 weeks I would put the bike in the car and drive part way to work. I would park the car in a supermarket car park and cycle the rest of the way. I started at about 15 miles each way and built up to around 26 miles each way. Getting washed and changed at work was not always convenient, but it did mean I could get some miles in during the week. On two occassions, I arranged to take half day holiday and work the middle 4 hours of the day from 10am til 2pm. This meant I could cycle from home to work and back, a round trip of 100 miles! On some weekends I managed to get out for 3-4 hours and cycle in local countryside.
Altogether I clocked up around 600 miles of training over 6 weeks, which is not that much really. I felt confident that I could cycle 100 miles+ a day but I think if you can manage 60 miles without discomfort then the other 40 miles require mental training rather than physical. In terms of aches and pains, it is important to get things right before you go on the ride. Dont be afraid to make adjustments to your set up, keep changing it until you feel really comfortable. I think I suffered with saddle problems and hand numbness because I didn't string 3 rides together. My body always had 2-3 days recovery before I got back on the bike. My advice is: do 3 rides over shorter distances when training rather than one big ride, it will probably show up more potential issues.
Make sure when you train that you eat well. Keep off fatty foods, make sure anything you eat has less than 25% fat content by calorific value. Be careful, as most manufacturers only show fat content by weight and fat has over twice the calories than protein or carbohydrate. ie:
1g Fat = 9 Kcalories
1g Protein = 4 Kcalories
1g carbohydrate = 4 Kcalories
You will probably need to eat more complex carbs like bread, rice, pasta etc to bulk up before a big ride. Keep eating at regular intervals to prevent the dreaded "bonk". I stopped every hour and had something like bananas, fruit cake, nutri-grain bar. Also drink plenty of water when training/riding and during the days in between to maintain good hydration. On the ride proper I had a good breakfast (not fried) and a good evening meal. I stayed off alcohol until John O'Groats, but those riding less miles in a day could afford a pint or two as their body would have time to take on more water before the next day, just be careful!