Hi, pedaling pensioner,
It's of course up to you to decide where to start the Camino de Madrid, but you don't have to go nearly as far away as Cercedilla to get out of the city madness. I know many people who start in Fuencarral, which is on the metro line, and a mere 8 km outside Madrid.And here is a link to the Metro's policy about bikes:
http://www.metromadrid.es/en/viaja_en_m ... ceso1.html
From Fuencarral you are well out of the city.
Others start at Tres Cantos, which is 28 km from Madrid. That would require you to get on the "Cercanias" trains (the commuter line), rather than the metro. And I'm not sure exactly what their bike policy is. But there is a dedicated bikeway all the way into Tres Cantos, so you certainly can start earlier if you want.
If you start at Cercedilla, you will miss the lovely totally rural stretch through countryside between Colmenar el Viejo and Manzanares el Real, and the castle in Manzanares is a show-stopper.
But if you want to start at Cercedilla, which is after all what you asked, you will have to get on the new Cercanias line at T4 in the airport (info here:
traveling-to-from-the-camino/topic11742.html) and take it in to town, probably to Chamartin station, and transfer there to the Cercanias line that goes out to Cercedilla.
My pictures from my walk this summer have some indication of how quickly things become rural, just in case you are tempted to start earlier than Cercedilla.
https://picasaweb.google.com/laurie.rey ... vqZ76-DEg#
I walked this Camino in late May and it was very lovely. Very solitary, but I really enjoyed it, and the pilgrim infrastructure is quite good. Check out some of the other postings on the details of this route if you are interested, there's quite a bit of info already on the forum. Buen caminio, Laurie