Hi, I was wondering whether there is a name for this phenomenon. Nowadays I often find myself considering all the things I need to do and I get overwhelmed and it is quite agonizing. But actually doing those things is either fun or not even nearly as bad as how it makes me feel when I think about having to do them.  
For example, I recently went to the beach with some friends and the beach is surrounded by the national park, we walked through the national park on that day about 40km. I did not find walking this distance to be difficult at all, I actually found it pleasurable and fun.  
However, I know for a fact that if someone would have told me that morning that I will have to walk \~40km that day I would have been agonized thinking about it all the time. How I have to walk 40km, how still so many km are left and so on. The mental strain of this would have been massive. While the actual experience of walking 40km is pleasurable.  
Even as I know this now, if someone would say let's do the same thing next month, even as I actually want to do it and enjoy it, just thinking that I will have to walk 40km would really negatively affect me. I observe this happens often with things I genuinely like. For example, I am doing an online course which I absolutely really like and actually enjoy doing, but the thoughts of thinking that I will have to watch this many hours of lectures this week and so on are absolutely flooring me, even though when I watch the lectures I actually enjoy them.

Is there a name for this phenomenon?

Thanks!!!!