More of the same and why does my heel hurt?

Another 8km sojourn around the suburb here this evening. This 8km at mid-tempo pace (4.38min/km) has appeared very regularly on the run program - typically once per fortnight. So I know the pace - and course - fairly well.

Knocked out the 8km in 36.36min which is reasonably similar to other efforts over this course. Maybe felt a bit easier than some so definitely improving.

Made me wonder, though, how would I go keeping up this pace for 21.1km? Guess I'll find out in a week or so.

Why does my heel hurt?
Ok, I probably know the real answer to this. It might have something to do with running more in three months than I have for each of the last few years? Sure, but why does my heel hurt so much???? Strangely enough, it doesn't bother me too much when running. Mostly only when I am barefoot. Running shoes give good cushioning so it is not much of an issue after 5 minutes or so. Anyway ....