DISCLAIMER: I have no scientific evidence to back up what I'm about to say. This is simply my interpretation of what my body is currently experiencing. If you can prove me wrong and have a better explanation, feel free to share and I will not mind. :)
Fitness is like a video game. There are different levels, and each time you reach a new level, some tasks suddenly get much easier, and you can start developing some new skills.
I feel like I reached a new level sometime in the past couple of weeks. My second 5K was much easier than my first, and the week before that I ran nearly 4 miles. Yesterday I completed a 5-mile run, and it felt amazing! I only had trouble breathing during the 3 challenging hills, and I did not feel like dying at the end.
It seems like just yesterday when running 30 minutes straight was a real struggle. Now 30 minutes is an easy run, and twice that is more challenging than painful. Although my overall progress has been taking place over a period of many months, it seems like the change in the way I feel has happened rather suddenly, and thus my theory of fitness levels was born.
Please do not worry - this does not mean I am going to take my current level for granted. I do not expect every run to go perfectly now, even the shorter ones. In a video game, you can still be felled by an obstacle you've beaten a thousand times if you're not paying enough attention, or something has happened to throw you off your game. Likewise, I could easily tank a short 2-mile run on a random bad day. The difference is just that now my perception of what is an "easy" workout has been adjusted upwards, and goals that once seemed out of reach are now just barely beyond my grasp!
You can probably ascertain from this post that my 10K training continues to go well. I still do not expect to run the whole 6.2 miles the first time out, but what I do expect is to enjoy whatever portion I am able to complete. I truly am addicted to running now, and I'm having so much fun with it! It's amazing how much more enjoyable it is once you "level up" and get to a point where running a few miles is pretty easy. I look forward to improving my distance and time as I work towards the next level!