Running marathon is not healthy. It is abusing your body more then is healthy for it. Additionally, people with zero relevant health problems for normal life can still have conditions that prevents them to run marathon. Whether joint normally-non-conditions or hearth normally-non-conditions or I dont know what.

Also, training for marathon requires a lot of time and there is opportunity cost there. I will not blame father for spending his time with kids instead of training for marathon or what not. Nor someone who spends time educating himself rather then physically training. Or reading book, washing dishes or just having time consuming job.

The motivational quotes are told to make people feel good.