You begin the ascent to Peñalara from the Puerto de Cotos, a mountain pass at 1,830 meters. It's only an hour in car from the center of Madrid, and can also be reached by train. We didn't have the highest hopes for our day--the INM predicted a 90% chance of rain and the sky was gray and threatening above as we began our uphill trek. Above the treeline, the wind began to whip against us with such a fury that I was afraid we'd be blown off the slope. But it didn't rain. In fact, we noticed as we got higher, the wind was blowing the ugly rain clouds away from us and onto the plains of Castilla-León. So we reached the summit quickly (it's a 600 meter ascent in a only a few kilometers) and continued on our loop down the other side of the summit and up a rocky pinnacle called Risco de los Claveles and down, down, down until we reached the first of several glacial lakes that would mark our return route.
The circuit hike took us about four hours not including stops. Be forewarned that it's extremely popular (like most of the Guardarrama). We had a lot of company on a day with relatively bad weather. That student of mine hadn't made it to the top because of worse weather. A steward at the Laguna Grande told us that two years ago there were seven deaths on the peak due to winter unpreparedness. Best times to go are May, June, and September.