I would also not run any mythic adventures until 20th level.
Additionally, any monsters that are CR 20 or less...ditch them. They don't count as mythic as far as i am concerned.
I mean come on...mythic OGRE?
When I hear the word Mythic I think of fighting Scylla and Charybdis, wrestling the kraken, punching out Chthulu...not fighting some dumb ogre.
Allowing mythic at low level was a huge design mistake.