Leon is roughly 300 kilometers from Santiago, and please don't sleep off your jet lag and run off down the Camino -Leon is a marvelous city the cathedral is a magnificant example of Gothic architecture but I especially love the Romanesque St Isodoro and yes do the tour even in Spanish it is amazing. Having said that you just lost two days walking and we haven't begun to talk about say Astorga or Samos! A day to get back home and you are looking at ten days +-. Santiago desrves a day or two, so you are rapidly losing walking days so plan well! Sarria to Santiago is not difficult, the countryside is often magnificant - the cities, well, much less so. Weekends on this stretch can be hectic since it is often used as an inexpensive walking holiday by the Spanish and it may be difficult to find lodging, you might wait to be seated to eat. I would see this attempt as a first course, a tasty something before you come back to Spain with enough time to do and see and exerience as much as your soul can absorb of a most wonderful moving and exciting encounter -first with yourself and secondly with the Camino and those people around you there.
PS The Camino in Galicia is also a popular passage for the local livestock, cows, sheep, horses etc-the mud you see on the Camino there is often not mud! You have been warned.
PPS Ponferrada is nothing to write home about. The church is more interesting than the Disneyland "Templar" castle.
S.