The significance of Madrid, apart from it being a major milestone through Spain, is that it is one of two antipodal points along Mark's route. According to the Guinness criteria, which you can read under the route tab on http://www.artemis.com/ , the route for a global circumnavigation must include two antipodal points.
Having taken a slightly different tack in today's posting, I think I might take the liberty of bypassing Madrid and fast forwarding to the last part of today's journey which took Mark into the foothills of the Sierra de Guadarrama. The highest peaks in this range rise to around 2500m (hence the snow as in the Sierra de Gredos which we saw yesterday) but his route tomorrow morning will take him across a col at around 1500m. However, in order to reach there, there were two tough climbs today. This is because, as you can see from the map below the highest ground forms a V shaped ridge with an intermontane valley. So, having ascended the first ridge (Sierra de la Cabrera) and dropped down into the valley, it would have been all uphill again!
Mark has ended the day today at around 1200m - high enough to produce a cooler microclimatic zone. These images, all borrowed from Flickr and taken within a few miles of where Mark is this evening, give an impression of the surrounding landscape ......
It could be quite cool up there!