Hello Grunt123,
Welcome to the forum!
There are some mixed reports about booking Renfe tickets online. I have never been able to do it - they require a PIN and I was told that the PIN is only issued if you are a resident of the EU. Now I'm not certain how accurate that information is, but I can tell you that I speak and read Spanish pretty well and have tried over and over to book tickets with no success. I've traveled many times by train in Spain and I've always had to go to a train station and book in person.
In terms of getting to the train station from Barrajas, you can take the Metro (subway) if you choose. Depending on how much time you have and from which station your train departs... I believe a train to León would depart from Chamartin.
I suggest you contact Ivar... he may be able to help you and possibly get a booking for you. The website link is at the bottom of the page.
As for starting in León, the Cathedral is not to be missed and the Camino picks up just outside the main door directly across the plaza. Just follow the markers through the old section of the city, past the ancient murallas (city walls) and out past the Parador San Marcos. As you cross over the Roman Bridge, the markers are on the ground in front of you - hard to miss! Many people stay an extra night in León. there is a great market there and definitely have a coffee in the Parador if you have the time - they have a beautiful sello!
Buen Camino,