The rain never appeared, but my umbrella seemed a natural implement with which to indicate the sights we passed. The idea of leading guided walks always appealed, and the publication of my book Walk the Lines (I walked the whole Tube system overground) gave me something approaching credibility. History blows their mind: they think 200 years is a long time, while we think 200 miles is a long way. London is the perfect city for walks because of its variety. The best walks are where the information flows both ways, and people tell me things I didn’t know.