Arriving in Barcelona..... return from Madrid

The town of Sarria is about 100km from Santiago. There is not bus service in every town, but there is regular bus service along the route.

Falcon pretty much summed it up.
You can catch a bus from some villages, but not all.
I have a 17 day trip that I host and our schedule includes walking from Pamplona to Logrono, bus to Burgos, bus to Leon, bus to Astorga, walk to to Villafranca, taxi to Samos, walk to Santiago. You can see our daily itinerary and schedule if you go to this link and scroll down. Something like this schedule might work for you.
http://anniecarvalho.weebly.com/2015-itinerary.html

I would bus an extra day or two and be sure to spend time seeing Montserrat in Barcelona and visiting at least the Prado and/or Sofia Reina museum(s) in Madrid.

I know I will get be beaten up for this post, but so what?

One question: Why do you want to do it that way? If you have only a limited amount of time - start closer to Santiago. If you jump with a bus or similar ahead you will every single time lose your newly won Camino friends / family.

Also 'bus into every town', I do give you the benefit of doubt, but just in case: If you plan to bus the Camino AND to stay in traditional pilgrims albergues, please be aware of the fact that you are taking a bed away from somebody that walked the whole stage without bussing ...

It is NOT a cheap holiday / vacation, it is a pilgrimage and that includes the nice and the boring parts, the good and the ugly.

SY *who is fully aware of being awarded punishment points for this post and gladly accepts them*

PS If I misunderstood you, I apologize in advance and profoundly!

Sonja Van Rhyn said:

Where is the last 100km that you must walk from?

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Are you interested in getting a Compostela? Otherwise, there isn't anything/distance that you must walk (i.e.: you can walk whatever part(s) of the route(s) you want). Even if you are interested in getting a Compostela, the location of the point 100 kms away from Santiago de Compostela will depend on the route you follow. You say you plan to start in SJPDP but Do you want to follow the French Way all the way till Santiago or do you plan, for example, to take de camino de invierno once in Ponferrada?

Sonja Van Rhyn said:

We have 18 days to complete the route and we still want to take time to enjoy the little towns and do some nice stops?

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What route? That's the key question. All what you bus is route that you aren't (really) completing... You can make part of the route by starting in a closer point to Santiago de Compostela than SJPDP, you can mix making part of the route with making some tourism in some cities/towns along the part of the route you didn't make this time or, with a little more of planning, you can make something similar to what you quoted on your original post but it all depends on the aim of your trip (e.g.: if you are more into pilgrimage or into tourism...).