Public Transport from Madrid towards Leon and Santiago

[bwalsh]

My wife and I are wanting to do a small introductory walk to Santiago
eg. 7 days or let's say approx. 200 kms east of S de C. Initially I
thought of starting at Leon but I now know it is 330 kms from S de C.
Can anyone suggest another place which would be suitable bearing in mind
we will arrive in Spain at Madrid ?
 And the follow-on question is : what form of public transport, out of
Madrid and towards Leon etc, is economical and recommended ?

Brian & Agnes