Some excellent advice already. If the chassis needs work then I’d add in some strength work to your routine. The individual daily distance and elevation is nothing extraordinary. Where the toll comes in, is doing it day after day after day for a bit over a month. Strength work will help with that.
As far as the riding goes include a mix and short and hard and long and easy. If you haven’t done a 100km ride, or you don‘t do them that often, then do some. Make it so that 100km feels comfortable for one day. It’s not a mental barrier or something that makes you nervous. You can go out and do 100km at any time. If you’ve got hillier routes include them, if you’ve nothing long do a few loops or repeats of some hills. Once you are comfortable with 100km in a day, try doing it 3 times in a row. The challenge of multi tour days is being able to recover each evening, so that you can repeat the distance and terrain indefinitely, without your speed or condition dropping off. A 3 day tour of 100km each will give you an idea and how you do when doing it day after day.
The way I get comfortable with hills is to go out and climb the hardest steepest or longest ones near me. I cannot replicate the length of hills elsewhere so I’ll do repeats or loop round a circuit. Similar to 100km holding no fear, you get to the point no hill holds any fear as you’ve done the very steep, and you done so many repeats you know your legs can handle long efforts.
Now onto the Alps, assuming you mean the European ones. The climbs will be long and but fairly steady. A climb might be 20-30km, and thus you’ll get the climbing in 3-5 climbs or passes per day. As above get suitably low gearing and you’ll be able to settle into it, at a comfortable effort. No doubt the company will plans lunches at suitable points throughout the day, giving a break say once every couple of hours or so. Riding in the alps is great.
Sounds like a great trip.