If you look at the lines that graph long-term athletic performance – VO2 max, anaerobic threshold, maximum heart rate, 10k running speed – it can be pretty demoralizing. Even before age 30, there’s a slow and steady cant downward, and then by the mid-40s, a descent into a valley that plunges toward the abyss’s void of emptiness. Some of us choose to oppose these trendlines, wrestling with them like tentacles in a roiling sea of...