So I did the first 30 min run today, and I optimistically thought I'd try going at a slightly faster pace, because I used to run 5k in under 30 minutes with no problem. It was going ok and I wasn't feeling exhausted or out of breath, but then around the 13 minute mark I started to get a sharp pain in my chest.
Now, after I stopped running (about 4 years ago I think), I tried starting it up again a year later and found that I was getting this same pain. There were a few other reasons to be concerned about my heart at the time, so got an echo of my heart done and the cardiologist said it was functioning beautifully and the pain was probably from over exertion (trying to run too fast/far too quickly).
So anyway, when I got this pain again today, it reminded me that, as much as I'd like to be running 5k in 30 mins, I have to listen to what my body is telling me, and clearly 13 minutes at that pace is my body's current limit. I slowed right down, but kept going and managed to finish the whole 30 minutes, which I'm really pleased about, but still slightly annoyed that my body said no, as it was going really well until that point!