Why running can be a pain and joy at the same time !

Recounting my run on 24 Nov 2013, Sunday, I fondly recall how I felt at each stage of the run! Started the at just below 6 min per km pace, early stage of the race normally will be ok. Picked up speed along the way, 2nd km was in the 5:50 range, 3km was like 5:45. Still feeling fine although my training had been erratic all year round. So still feeling ok even at the 5km mark, which was around 5:40 range. Now cruising along the 5-6km long straight stretch, still feeling good. Once pass the 7km mark, sign of fatigue start to set in. Completed the 8km in 5:16, which is fast by my level of fitness, start to slow to around 5:50, eventually timed at 6:02 at 9km. Covered 10km in slightly under 1 hr, the km split was 6:27. My plan was to stick with my buddy until about half way mark and then run at my own pace. I was thinking to himself that I would slow down after the 10 km mark, which I did but luckily I did not slow down too much. On that day, I was feeling pretty apprehensive as my training was way off the mark. At the 11 km mark, I was slow but did not feel as bad as I thought. I just continue at a slow but steady pace of 6:05 to 6:10 pace. When I was left with about 6km to go, I was still feeling pretty good to my surprise. I was quite happy that I was running steadily and tried to just run easy. With 2km to go, I was quite sure I am going to finish comfortably, I tried to increase my speed a little and eventually finished the last km in about 5:38. Completed the 22km run in 2hr11min43sec, much faster then I expected.