[Oyk]

To keep fit, marathon is not advisable to the elderlys. Brisk walk or slow jog 2.4km enough already.

Depends on what you mean by elderly, what is the cut off age.

I would not even recommend a slow jog.  You not only have to be mindful of your heart, but also your knees.  The joints there had been supporting most of your body weight for years - your head, your torso, your arms, your buttocks, your thighs...that's a helluva lot to support....and when you are elderly, it's not a good idea to still pound on those knees and wear out the cartilage, or whatever is still left, even more.