I have to agree with William: Leon is a good place to start for your average 'fit' walker.
You of course have to gear it to your personal abilities and the prevailing weather. Injuries incurred (blisters, twisted ankle) can put a real crimp in your walking style. See if you can build in a couple of buffer days to make sure you will make Santiago in your time allowed.
Astorga is a reasonable altenative if you wish to take a slightly more leisurely approach to the Road. Both train (RENFE) and bus lines service Astorga, so it is easy to get there.
Villafranca del Bierzo could be the next starting place for your Journey, but I don't see a train stop there. Ponferrada, however, is serviced by RENFE. Both have bus service in and out.
Some folks like to start in O Cebreiro, but it is not as easily accessible. There is bus service to Pedrafita do Cebreiro, an easy 4 km short of O Cebreiro.
Watever you do, whatever you decide, have a great Camino.