Saying I run for fun is a very romantic perspective, but it's not 100% truth.
It's easy to love running when everything is going well in your life, when you have new shoes and the sun is shining outside. However, those who really love running are those who run even when they're not in the mood, at anytime they find a gap in their agenda, in the dark, cold or warm and, therefore, those who sometimes even dare to hate running.
Every now and then I run just because I have to. Off and on I have to go out without thinking too much why or where I am going, and sometimes I just go because someone is counting on or waiting for me.
It's barely impossible to love running when you had a hard day at work but you still have to accomplish the f*cking schema you made when everything was just fine. It's hard to enjoy when it's too warm or to have fun on the first 10 minutes when the temperatures are bellow zero, for instance - and people who run just for fun certainly don't go out in those circumstances.
On the other hand, I NEVER heard anyone regretting after a running session. If you regretted you did something wrong, trust me.
Running was always my second sport and I was used to run 5 ou 10Km per session and a maximum of 40Km per week, but 4 months ago I decided to run a marathon. As a Sports Coach I knew exactly what to do and did it. Two weeks ago I ran my first marathon and in two weeks from now I will do another one - something I would never recommend to a coachee, by the way!