IME it takes a long time to get fit. About 7 years from starting to ride to as fit as you could ever get. So start now. Start working your weekly mileage up to around 150 or so. This should take some months. Then add things like hill repeats, group rides, other hard riding as you are able. Ride 52 weeks/year. Plan to ride a couple of double centuries next summer. If you live somewhere that's not possible to do outdoors, buy a set of rollers and join a gym. Besides getting in shape, this will give you time to resolve bike fit and saddle issues. Meaning buy the bike now that you'll plan to use on your trip and start riding it. Since yours will be a fully supported ride, you won't want a touring bike, but rather a TdF style road bike, probably carbon.