What is the good exercise running or walking?

Walking better,,,: Although running can be effective in the short term, it's hard to maintain if you're not used to it. It's better to start with a light walk, then make it more brisk according to how much it can be tolerated, and finally jogging within reason. This helps maintain a long term effort, and reduces the likelihood of joint damage.

Different for each: For some people one might be better than the other - but you are right, activity that stretches your normal limits is good for us. If you are careful not to overdo it and to avoid injury try each and discover which you like better. Best wishes!