I always say that running makes me feel very happy – and it does! No matter how hard sometimes it is – it makes me very happy. I feel so great if I do a very hard training (of course, during the training I am just waiting for the end:D ). Hard training is one part of success. You have to prepare for the battle (competition). As more you prepare, as more easy it will be during the competition.
I don’t want to scary, but yes, sometimes running is very demanding. Well, running is the most efficient sport when burning calories, so you need to sweat for that, work really hard. I am not talking about simple slow running, I am talking about fartlek or tempo running, or very long weekend run.
One thing you have to do is not to give up. If you feel very tired, do not run too much next day or skip one day (but not more! (unless you have injury)). Just listen to your body, it’s very very important. If you are beginner – do not push it too much. You have all your lifetime to do as many running training as you want, as many competitions as you want 
One very important thing for beginner is not to increase your mileage too fast. I believe that after two weeks of running 20 or 30 minutes slow running is very easy, but do not rush to increase your running time too much. The rule says that you should increase your mileage about ten percent per week. So if you did 40 km last week, next week you should run not more than 44km. This way you shouldn’t get any injuries, because sometimes a lot of beginners’ muscles are not ready for long distances and they get really serious injuries.
If you do not hurry up in the long time you are going to win. If you rush too much, you get injury, and you have to skip one month or even more. So better to do it slowly and run all the time, than to skip a lot of time.
And just after you feel you are ready and strong enough – include fartlek and tempo running in your training. After that you can include fast running uphill and very long weekend runs.
It’s better to have that straight line to your success than the second one which takes a lot of time, disappointments and injuries
Good luck!