The word "long" can imply many things. Long is used in the temporal sense, indicating something that lasts for a time that is greater than the average duration. Long events, e.g. a long speech, can get really boring and sitting through them can be quite a task for people.
Similarly, long is used in spatial sense too. In spatial sense, the word indicates something that is greater than the average spatial extension. Long can also be used to describe any entity of relatively great height. Things which are long occupy more space than things which are short.
The word "long" can also be used to describe someone's craving or yearning for something. When you long for something or someone, you desire it strongly and persistently. When you are "long" on something, you have that thing in excess. For example, someone who is "long on brains" has more acumen than required.