For me personally, a long run is anything over 90 minutes though if I train for a marathon it increases to two hours. They are important because your body needs to get familiar with the feeling of fatigue and they are a building block to your endurance. They are very rarely fun, at least for me. I find long training runs hard. That is partly the reason why I try to sign up to as many races as I can during a marathon training. To have the support of the crowd and the company of other runners makes a lot of difference. Even if the races are de facto just training runs. The goal is not to achieve a new PB – they are about practicing my pace, I am testing the gear and planning the nutrition for the marathon day. And they are way more fun than running on my own.