Activities to increase chances of getting into business schools

GMAT and essays are very big components of the application, so you should spend enough time on these. That said, b-schools do look for well-rounded profiles and if you do not have any current extracurricular activities, it will be worthwhile to build upon these, even with limited time in hand.

Whether that should be marathon running or something else depends entirely upon your interests. Purely from the admissions perspective, its more meaningful to do activities where you alone are not the beneficiary, but where you positively impact other people. That could be a volunteering activity (your organization's CSR could be a good place to get started). Also, if you are only starting now, its better to pick up an activity where you already have some experience. That will help you show some continuity and genuine interest.