Andrew, if you're wondering about which would be a go-to English translation of "Él era un buen corredor"... it is my best estimation (of course I'm still learning) that it would just be "He was a good runner".
But if the desired meaning in English is "He used to be a good runner", I believe a Spanish speaker that is being extra conscientious to avoid any possible misunderstanding might say something like, "Él solía ser un buen corredor".
However... as I'm learning more and more... Spanish speakers depend much more on context, so the sentence "Él era un buen corredor" could actually be used to mean "He used to be a good runner"... [pero ya no]... relying on context for that nuance.
Kwiziq staff / others will hopefully chime in further to provide a correction or more context, if needed!