Tour De France-Training regimen for these fitness maniacs?

Granted, all the Tour de France Cyclists have their own training programs/team programs or whatever. I just simply find it hard to comprehend how they get to these fitness levels. Can someone break down for me a vague approximation of the amount of training-mileage/duration/intensity-and torture that these cyclists undergo to take on the ultimate bike ride?
I am especially curious to find out the average cruising speed of tour de france cyclists.

All the riders are professionals, they will all have started riding a bicycle seriously, in their early teens, racing in their local club's amateur races, they then progressed to larger amateur races where they probably excelled and built a good
racing record of high placings, they then applied to or were invited to join a professional team, and eventually got the chance to ride in a Classic like the Tour de France.
So you can see that, for many of them, they will probably have been racing, building strength, stamina and ability for maybe 10 years or more before they get the opportunity, and it is not so much the training they now do, but the work they have put in over the years, now it is more maintenance training to keep their form. A large percentage of the riders have been racing as professionals for a number of years and much the same applies to them. They all have to ride every day, even on their rest days, or they are likely to 'sieze up'.
This year the longest stage is 230km and the total for the 21 stages, including time-trials will be 3554km.They will average around 40kph for the whole distance, some stages obviously being at a much faster pace.

Most importantly, I forgot to mention that, no matter how hard the training is, nothing can compare with actual racing, so they will ride a lot of one and two day races, starting early season and probably stage races like the 7 day Paris-Nice which is on at the moment, up until very shortly before the Tour starts. Some of the old stars, like the greatest cyclist of all time, Eddy Merckx raced for more than 100 days a year, he won the tour five times, that was really before teams took over and controlled the racing.

Just lots and lots of miles. 5-7 hours a day of riding, one rest day a week. Lots of hill climbing & sprint training. Sensible diet.

During the race, they will average about 25mph over a 5 hour stage. Flat stages are a bit faster, mountain stages a little slower. Time trials routinely exceed 30mph. The final sprint to the finish can exceed 40mph.

Pro tour level racers train by committing suicide several times a day, and eating entire supermarkets. This enables them to cruise at about six times the speed of sound.

500-700 miles per week in training miles. Regular motorpacing with a motorcycle or scooter.