Walking

Walking is a slower paced activity than running, and most people are able to do it.

Pros:
easy for most people to do with no specialized equipment or training and open to all ages, easy to do while talking with groups, relaxing enough for most people to enjoy the scenary while participating, not very likely to cause injuries compared to most physical activities.

Cons:
Probably not as suitable for competitive people as competitions are pretty rare in most areas, slower to burn calories than more intense activites, slow and moderate paces may not give full workout (power walking may be necessary for fit people). Also, it doesn't really teach, or hone you a special skill like most sporting activites, as most healthy people are already able to walk for long periods of time if they need to (however, if you aren't it's great to get this ability).

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The pointless ocean moans on top of each conventional mathematics.

Running

Running is fun and easy if you are in shape as long as you don't live in a community that is too polluted

Pro: easy to get involved in organizations or clubs, burns calories quickly, cheap (only shoes required)
cons: tiring, less chance for scenary enjoyment than cycling, not for those too far out of shape