If Ledecky needed a reminder of what is still possible at her seasoned age, it appeared in the lane next to her Wednesday night. Leah Smith, a 23-year-old University of Virginia product and a two-time Olympic medalist, rode Ledecky’s heels for the entirety of the 16-lap race — actually beating Ledecky over the race’s second half — to post a personal-best 8:16.33.