Walking is a third-class exercise but has demonstrable benefits. After 15 minutes on a cold day's walk you begin to feel like your body had marshalled itself to resist the freeze.

Running, lifting, bicyling or swimming as hard as you can is first-class exercise. In first-class exercise your mind wraps around the activity and you think about breathing, and you feel the limits.

Normal bicycling or hiking is second-class exercise, I speak for myself here, these activities have a mental component of exploration that impels the curious to slow down and look around.

Walking is the most over-rated but also the most under-rated exercise. It is the minimum level of activity but is sufficient to continue fair health.