When Brendan Leonard decided to get serious about running in 2015, it only lasted a few weeks-the "getting serious" part, not the running part. Since running his first-ever ultramarathon race that year, he's logged thousands of miles on roads and on trails, completed dozens of marathons and ultramarathons, and spent a lot of time thinking about why we do what we do when we put on running shoes-whether we're qualifying for the Boston Marathon or...