Hard is the opposite of "easy." If you're just beginning an exercise routine, don't push yourself too hard in the beginning, to avoid injury.
Hard can mean "difficult," but it can also mean "rigid." A hard surface is one that doesn’t give way, like stone. If things are tough for you, you might tell someone you’re having a hard day. If someone gives you hard liquor, watch out — that’s the strong stuff. If you’re a teacher, you want to make your examinations hard enough to be challenging, but not impossible. If you’re stubborn, people might call you hardheaded.