I would love for the Personal Records page to automatically detect, or at at least allow for, a PR within one of my runs.
For example, just now I've done a fast 4 mile run. Within those 4 miles I've achieved a new personal record for a 5 km run, but if I were to pick it for my 5 km PR, the PR page shows the duration of the entire run, which is of course a lot longer than whatever 5 km runs I do. So, if I want to compare activities PR-wise, I can only do so if I run exactly 5k, or exactly 10k.