I think that from Madrid to Cercedilla there's not any problem for cyclists. I'd recommend leaving right from the Plaza de Castilla, because a lot of that way into Tres Cantos (where many people start, taking the train to Tres Cantos from Madrid) is on a specially dedicated bike path.
From Cercedilla up to the pass at Fonfria, about 800 m of elevation gain, has some stretches on the Roman Road. You would have a tough time cycling on it but I wouldn't think you'd want to miss the opportunity to see it (and the two roman bridges on the way are petty incredible) -- I just walked Cercedilla to Segovia last week when I was in Madrid for a few days, and it's really a knockout stage. From the pass down to Segovia is perfect for cyclists, in fact I saw a bunch of them on the other side of Fuenfria. Cercedilla to Fuenfria is about 7 kms, and there are plenty of parts of that stretch that are perfectly cycle-able.
My journal says that from SEgovia to Santa Maria is where we hit a few kms on big loose rocks, followed by sand in pine forests that you sink into. But this stage is never too far from a road, so I imagine the workarounds are easy. There are other big stretches of pine forest (like going into Coca), but if my memory is right, it'd be fine for biking.
In general, I'd say this is a great camino for cycling, maybe even better than the Frances because of where the paths are -- many many kms of agricultural tracks and forest paths, little elevation gain. I think there are very few sections where you would have to leave the marked Camino, and since you're never far from roads it can't be that hard. Once you hit Sahagun, you'll be overwhelmed by the masses on the Frances, which will probably make you appreciate the Camino de Madrid even more. Buen camino! Laurie
p.s. I know I posted a link to my pictures, but in case you haven't seen them, I think they'll give you some idea of some of the terrain.
https://picasaweb.google.com/laurie.rey ... vqZ76-DEg#
