Why walk the last 100km
The Majority of walkers along the Camino de Santiago walk the last 100 kilometers. It is a matter of time mostly that keeps pilgrims from the longer walks, although, many return to walk the entire route after experiencing the last 100 km. By far the most popular starting point is Sarria on the Camino Frances, although Tui on the Camino Portuguese is becoming very popular as well.
There is no set starting point for any Camino. You are free to walk any or all of it. Starting in the middle or just walking from the beginning to a point. There are many pilgrims each year that walk a part of the Camino and then return at some other time to either complete the Camino or continue until they must return home. It is possible to complete a Camino over several years using just one credential.
the last 100km
To qualify to receive a Compostela, you must walk at least the last 100 kilometers, or you may bike or ride a horse at least the final 200 kilometers. You must make sure that you have at least 2 stamps (sellos) for each day that you walk in the last 100, or 200 kilometers respectively.
The majority of people choose to walk, do so from Sarria on the Camino Frances. While that is a great choice many other options are less crowded and quite beautiful.
There are some very beautiful sites along each way. Consider trying an alternative route.