So the author maintains that fitness itself is not enough. We need an exercise regime that improves:
- strength.
- stability.
- aerobic efficiency, and.
- aerobic threshold (VO2max).
Training for a marathon once a year should get you the last 2. For strength and stability runners need to incorporate some kind of cross training (which is a good idea anyway to prevent injury). Swimming is my go to sport for cross training as it provides strength (doing sprints) and stability (builds core muscles) as well as promote good breathing technique. And it's low impact so your joints get a break from pounding the pavement.