Route Napoleon is no problem, providing . . .
For cilento:
It took me 7 hours (actually closer to 8 hrs) from SJPDP, but I was deliberately not hurrying (I had a recent tendinitis problem training at home and I did not want to risk a repeat).
Some people do indeed report long days, but it depends on several factors: (1) What load were they carrying? I carried 7 kg, 10% body weight. Many people carry far too much and this can slow them right down, especially on day 1. If had to stress one thing re preparation, it is to be absolutely ruthless in cutting down weight - see my other posts here re packing. (2) How old were they? a surprising number of 60+ people (including me!) do the Camino and we may not be as fast as a 21 y/o. But then again "doucement mais surement" will get us folk to the end of the road. (3) How much preparation did thay do? If a person has not proved beforehand that they can walk say 30 km with a load over broken terrain, they may well get a surprise on Route Napoleon.
As for injuries, the most common injuries I saw over my 33 day camino were tendon injuries in the ankle. Tendinitis (inflammation of the Achilles tendon connecting the calf muscles to the heel) were quite common. Inflammation of the tendon connecting the front of the ankle to the shin muscles was also fairly common. Such injuries arise from overuse and will probably show up after some days, not on day 1. Bear in mind that in walking 30 km over hilly terrain you will take over 60,000 steps!
Knee problems imply some kind of pre-existing problem. So, I would advise pilgrims to get a professional chekout of their ankles/knees before setting out. I have been active all my life and have done lots of walking in the Himalayas and Andes. But when I did 120 km training over several weeks for the camino, I discovered (for the first time in my life) that I had an over-pronation problem that caused tendinitis.
Also, make sure your footwear is appropriate (and 1 size too big). There is a heap of info in this forum and on the net about this topic. The only boots I would suggest not taking are the traditional, heavy leather hiking boots. Modern shoes and light-weight boots are far better for many successive days of 25 - 30 km walks. I found that the camino is a totally different walking experience than trekking in high mountains.
If you are properly prepared, I would not worry too much about Route Napoleon. Remember that walking in July may be hot, so plan on carrying 2 litres (2 kg) of water and drinking often. Snack on high-energy muesli bars, bananas or dried fruit as well. Rest with the pack off every 2 - 3 hours.
As someone else said, most people take that route and (like me) they are pleased that they did so. If I had started in Roncesvalles, it would have always nagged at me.
I hope this is useful.
Bob M