I have done almost exactly what you mention, coming from about 20km MTB ride on Sundays to a multi stage, 100km aprox per day road ride, It was a 7 day supported ride, which meant the big suitcase was put in a bus and carried for us while we cycled. This ride totaled almost 600 km over the week. The first 3 stages where mostly flat with little/short climbs, starting at sea level, 4th - 6th stages had biggest climbs and where mostly ascents.
I did not train for this ride, just tackled the challenge.I took part on this ride four times, happening once per year. The first time I was able to ride the 100 km in about 4 hours. The biggest problem I had was wrist pain and saddle soreness. The body part that hurt most where my wrists. Muscle tiredness was nothing compared to wrist and hand pain/soreness. Saddle pain was mostly tolerable.
I was wearing padded shorts and using a bike that fits me "perfectly" for 3-4 hour MTB rides, but I was not accustomed to be seating for so long. Funny enough, I could do 8+ hour MTB rides no problem, but in MTB you do a lot of weight shifting, so you unload the saddle very frequently, and also, (rear) suspension helps a bit. In contrast, on this ride I used a hardtail bike with straight handlebars.
In about one hour after finishing each stage and having a meal I felt energized again. We also had the whole second half of the day for "recovery". so I felt almost as fresh as day one on the beginning of all stages. I can not say for sure how additionally tired I was after 4th - 6th stages, as they were significantly harder than the first ones.
On following years, besides having the experience of what it takes, I made a few changes:
- Used a handlebar that allowed more hand positions
- Tweaked bike fit a little to put less weight on my hands without being too upright.
- Fit the bike with 3 bottle cages and a small rear rack. 2 Bottles for water and one with a sports drink or sweetened drink. The rack for tools and cereal bars. The aim was to have all extra weight on the bike, so my body was free of any of it.
- Developed a "schedule" for drinking and eating during the ride which kept me feeling energized for the whole ride.
This last point I think made the greatest difference. What worked for me was: drink every 15 minutes, and have a bite of food every 30 minutes. The "food" was my own recipe of "granola bars" (About 10cm x 2.5 cm x 1 cm, so, kind of similar to commercial sizes. Ingredients where nothing fancy, just wanted to avoid extra sugar) By the end of the each ride I had consumed near 2 litres of water+juice, and about 3 granola bars.
The idea was to keep hydration and sugar levels fairly constant though the ride by sipping a little of sugary drink between along with water, and eat "slow carbs" that is, carbs that take a while to get into the bloodstream and remain there a little longer than plain sugar.
Another key factor for me during the ride is pace. It's kind of difficult to compare or convey how I distributed my effort, but I could say that my cycling is the analogous of really fast waking or slow, long distance running, as opposed to a 100 meter sprint. I was in the non-competitive group, completing the distance in 4 hours, approximately, where the elite competitors where finishing in about 2 hours to 1:45. Another way of saying it is that at almost any point in the ride, I could always have been a little bit faster, but I was "holding" somewhat. My only goal was "to get there" on time for lunch and keep the ride fun.
Related to that, was the use of an odometer or distance trip meter. Knowing the traveled and remaining distance made a positive difference in my mindset. When you don know how much on an effort is remaining, the mind plays a trick and makes you feel more tired than you really are (I think is an instinct to preserve resources). In contrast, when you are familiarized with a route you feel more energetic the whole time. In that regard having the cycle computer meant that even if I was not so familiar with "riding" the route, seeing the number kept me aware of my progress and felt more relaxed even when pushing harder than the first time.