For this purpose, and given this is may be a first time for some in your group, two starting places are most popular.
On the Camino Portuguese, starting just over the river and in Spain, is Tui. I believe the official distance from the Cathedral here is 109 km, someone correct me if I am wrong.
On the
Camino Frances, the proverbial spine of the Camino routes and most popular route, the usual starting point for folks seeking to walk the basic 100 km requirement is Sarria. I recall that the official distance from Sarria to Santiago is 118 km. Again, someone correct me if I misstated this.
Both starting places are accessible by bus or train from larger Spanish cities. Check
www.renfe.es and
www.alsa.es for rail and bus schedules. If this fails, go to
www.rome2rio.com. Enter your starting place and destination, and multiple choices will be shown.
Each route, from Sarria or Tui takes about five decent walking days...do the math.
There is a third route that is gaining in popularity. The Camino Ingles starts at Ferrol and ends in Santiago. I believe the official distance is about 111 km. Someone correct me here if I am wrong. Currently, there are fewer services for pilgrims along this route, but it improves each year.
If you do elect to do this remember that you must have a pilgrim credencial (available here from the Casa Ivar store). You need the credencial to use albergues and to establish that you walked the route to qualify for a Compostela when you arrive at the Pilgrim Office in Santiago.
You MUST obtain two sellos or rubber stamps daily to prove you actually walked the distance. Most folks obtain one stamp from the place the spent the night, and a second stamp during the day at a bar / cafe, etc. It is actually very easy to do.
Rules are rules. Unfortunately at some times of the year, and among some demographics, there is a tendency to cut corners and even cheat. True, they are only cheating themselves.
The folks at the Pilgrim Office in Santiago are very scrupulous about this, especially for pilgrims who only walk the minimum distance to qualify for a Compostela. However, and typically, if a pilgrim walks from France, Lisbon, Seville, Madrid, or from a further point of origin, they will make exceptions for occasional lapses in the two-sello per day rule for the final 100 km.
We all hope that you have a safe and happy Camino, and we are here to answer your questions and provide whatever advice and assistance we can.
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Hope this helps.