Maybe the mountains had grown. Maybe the Earth’s gravitational pull was stronger than before. Or perhaps there was something wrong with my legs. My muscles felt not just weak but empty. Richard Askwith is obsessed with hills and mountains — real and metaphorical. (The word “Obsession” even appears in the subtitle of his 2004 book “Feet in the Clouds,” about trail running.) But the Englishman’s fourth tome, “The Race Against Time,” is stunning for his climb through the clouds to seek the wisdom of true-life gurus — elite masters runners, trainers and scientists. Instead of asking the Secret of Life,…