After reading through the adventure path I realized that the actual source of the mythic power for the heroes is quite underwhelming. It is the rest of the exploding ward stones.
How this leads to them having to do mythic challenges in order to rise in mythic levels never made any sense to me, so I flavored that up.
Now, in my specific campaign, the wordlwound actually exists because the players failed in the campaign before.
So what my players are actually challenging is the combined lifeforce/souls of the former campaign area, represented by the voices of their former characters. So all mythic challenges I do opens them up to more souls/powers/followers that grant them their mythic powers, and I try to link their mythic challenges to something there former character would expect of them/be impressed by. To make it a bit more obscure, I shuffled the characters around, so the characters of other players are guiding new players.
My master plan is for them to gain sooner or later the commune with power ability, and then the players can coach each other from their old PC's point of view!
I am still looking for the equialent of their enemies gaining mythic energy, even though "powered by the essence of fallen demon lords" is already a bit of a parallel.
So, needlessly long intro, how do you play the mythic energy source? What do you do when a player chooses commune with power?