These are tidy, prosperous commuter towns with business parks, wide roads and a lot of roundabouts. Tres Cantos was a planned new town of the 1970s, a bit of an experiment in suburban living, and it has more green space than you might expect, thanks to the surrounding Monte de Viñuelas and woodland.
San Sebastián de los Reyes has the real local colour. At the end of August, the town holds its famous fiestas with bull running through the streets, the encierros, which compete with Pamplona in popularity for those who like their Spanish traditions loud and slightly dangerous. The rest of the year it is calmer, a town of low houses and a small old centre around the parish church, where you can find a good coffee and a decent plate of eggs and potatoes. Alcobendas is bigger and more corporate, home to the offices of many international firms.
The reason this makes a finish for a Sierra de Guadarrama ride is geographic. The A-1 motorway leads north from here toward the Somosierra pass and the mountain wall that separates Madrid from Castile, and on a clear day you can see the snowy ridges from the car parks of the shopping centres. That means when you come back from a day in the hills, the last bit is straightforward and quick, a fast run down off the sierra with lorries in the right lane and plenty of space to settle into. Fuel is no problem here, with several large stations and a reliable supply of cafés.
The only caution is the evening commute. Traffic backs up on the A-1 and the M-607 on weekdays, and weekend returns from the mountains are just as heavy, so a slightly early or late finish pays off. In the quiet of the hour after the rush, the sky over the sierra turns a soft violet, and the last twenty kilometres feel like a gentle wind-down.