@AnnieG , kia ora.
The question is how long is a piece of string? Or relevant here, how fit, in mind and body, are you for walking 21 days, day after day?
If you are up to it, it is possible to achieve Saint-Jean in 21 days walking at an average of more 35 km each and every day!
I was not certain whether the 21 days in your original post was from home (Nepal) back to home also included air and ground travel to Le Puy and back from your end point. This aspect might consume five or six days including ground travel and stop overs caused by a need to wait for the next connection.
If travel to and from is additional then that makes for a more ambitious intention. For example, consider these hypothetical calcultions.
stge totl days at
21 km per day
000 000 00 Le Puy
416 416 20 Moissac
088 504 04 Condom
082 586 04 Aire-sur-l'Adour
------ ------ 28 days total
stge totl days at
27 km per day
000 000 00 Le Puy
416 416 15 Moissac
088 504 03 Condom
082 586 03 Aire-sur-l'Adour
------ ------ 21 days total
A fall back process might be to aim for Condom or Aire and to make an assessment at Moissac of how far to continue.
Have you researched ground transport from Moissac, Condom and Aire onto your departure point from France? You might find that to be a limiting factor.
Depending on your interests you may wish to take half or full day stops at relevant places along your route, or even adjust your route to take them in.
There is nothing sacred about the route as such. You will have heard the expression "your journey is your journey". So there is no accountability to anyone for why you chose to walk other than the designated trail.
I would be pleased to continue this conversation.
Kia kaha