Something interesting to note (for me, anyway)
When I had been running 5K for a couple of weeks, I still felt like it was so tough - a struggle - difficult. It felt exhausting. But a friend said to me "running is 90% mental." I thought about that the next time I was on the treadmill and I realized something very surprising. Half way through the run when I was moaning and complaining to myself, I realized I wasn't actually tired. My legs felt just fine. In fact, I felt pretty good "physically" and I certainly wasn't exhausted. I was just moaning to myself because I "thought" I was tired. I "assumed" I was exhausted. But my legs definitely were not, and I was not out of breath. I realized that now I was getting used to running, the complaining was indeed mental. I was so used to assuming I was tired and it was a struggle, that it has just become my mindset.
I changed that mindset, and it suddenly became MUCH better. I'd repeat affirmations to myself like "running is SO easy for me" and "I feel SO awesome when I run." And everything just kinda changed :-)