Short Version: If you can walk 5 miles or up a long flight of stairs without breaking a sweat then you’ll manage on normal roads even if you find it a little difficult. If you can’t do those things, or would like to get into good shape so that the ride is easier then keep reading:
Most people I speak with about cycle-touring seem to think that you need to really fit to participate, and often the people I invite on tours are concerned that they will slow everyone down. Let me reassure you this has never been the case. Everyone I’ve cycled with has kept up without a problem and that includes non athletic people and even smokers, although I should mention here that I’ve never cycled with anyone older than 26 years so age may have been on their side! Nevertheless fitness is not usually a problem, just go a little slower utill your body adapts.
To make the ride easier from the start a couple of weeks training will help; a short ride, say 10 miles every other day just before the trip, building up to perhaps a couple of 20/30 mile rides at the end and that’s it. Then once on the trip, after a couple of days of cycling with good food and plenty of rest in the evenings most people will adapt to their new routine and very quickly be able to cycle effortlessly on normal road surfaces.
The only common health problems people face on tour are injury, such as knee pain and saddle soreness. Both of these can be prevented by gradually building up to larger rides and in the case of saddle soreness choosing the right kind of seat. Women, in my experience, typically benefit from a women’s specific saddle.
Another way to prevent injury is understanding cadence. When pedaling on a bike the rate that you push the pedals around with your feet, the speed of the crank rotation, is called the cadence. I find it useful to compare it to a tachometer on a car measuring ‘revolutions per minute’ (RPM). Cycling at a high cadence (roughly: one rotation per second or higher) will reduce the strain on your joints and the rate in which your muscles fatigue. It will allow you to cycle for longer and help prevent injury.