It is a solid fourteen days from Leon to Santiago (yes, I know there are those who have done it in a week), so Lugo or Sarria would be easier if you are concerned about your level of fitness. If fitness is not problem, then Leon is an excellent starting place.
I am not sure what a self-guided tour operator brings to the table except an end to your possible insecurities about organizing the walk yourself. Tens of thousands walk the Camino each year with nothing more than a guidebook. If you want advance reservations at hotels or a baggage service, then the tour operator will be doing a little something, but you can arrange your own when you get there by copying phone numbers from the nearest bulletin board, and making two phone calls daily (or having a tourist office make the calls). The routine is not complex: get up, walk, find a bed, and follow the other pilgrims to a restaurant. Repeat daily for two weeks. Go to the airport, and go home.
I recommend that you do whatever makes you the least anxious. Anxiety is one of a half-dozen things that can ruin a Camino, so avoid it.