If it is something you have been doing for a while you will not feel mentally drained. Of course it happens but when you are used to it, it will be very much interesting to you. I actually enjoy posting on forums.At a point, I am even tempted to believe that mental work saps much more energy than physical work. I don't know if you have experienced this too. When you are making posts online for three straght hours, you would be so exhausted and sleepy.