I'm currently running a Mythic E6 Legacy of Fire game, converted to Pathfinder rules. We are about halfway through "book 5" (I swapped books 5 and 6) and the campaign has gone very well. The story almost demands using mythic.
It was obviously a total rewrite from the ground up. Having converted other APs to E6 (Runelords and Serpent's Skull), adding mythic actually reduced the amount of conversion required. The addition of mythic offset the level disparity caused by using E6. The PCs are currently 6th level (+8 feats) & 4 mythic tiers (APL 12-ish). By game's end, the PCs will be 6th level (+20 feats) with 5 mythic tiers (APL 15-ish). I've found in practice that 1 tier = +1 APL/CR.
For most encounters, I used non-mythic monsters. I tried adding a mythic template to monsters in a few encounters - it worked but the monsters didn't seem that different from the "standard" (PCs didn't notice). I saved true mythic monsters for select villains and it really made an impact (PCs had to change undies).
I don't think every AP is suited for a mythic conversion. Mythic makes the PCs almost comically powerful and so much becomes utterly trivial. Mythic also speeds up the game: shorter fights = more progress each session. We've taken to cutting sessions short to not burn through too much material in one sitting.
To the OP - it can be done, but it requires modifications and not every story is suitable.