I recently started playing an Aasimar, have the feats Angelic Blood and now Angelic Wings. Why on earth does Impossible Speed not apply to natural fly speeds? I know that RAW it only applies to base land speeds, but, if i have a natural fly speed, why my new fastness not apply to that? Or was this just an oversight? And if we all rule that it cannot apply to flying, how in the world can I get my natural flight faster? Birds get to fly faster than they walk, and I believe a druid archtype will as well. Any opinions?
For whatever reason, Impossible Speed only applies to base land speed as stated. I'd suggest getting a copy of the Mythic Heroes Handbook and the Mythic Character Codex to give you more mythic options for your character. With Pathfinder First Edition being a closed system, those two books are the only expansions on the Mythic Adventures ruleset that we'll ever see. That said, they are well done and should be used by anyone using Mythic Adventures at their table.
Because the rules text says so. If you're wondering why the design team wrote it that way, I couldn't even guess. It doesn't apply to swim speed, climb speed, or burrow speed either. These all existed (and most were commonly available to PCs) when the ability was written, so it's not like they didn't think of it.
Haste still works, though, including (somewhat ironically) the effect of boots of speed.
Mythic Path options are for player characters. And player characters don't usually have their own fly speeds. You can call it an "oversight" if you want, but it's a corner case of a corner case, so it's not surprising. If you want to make your fly speed mythic, you will need to rely on house rules or third party content.
Note that the Agile Mythic Simple Template increases all movement speeds, because it's intended to apply to creatures that have their own alternative movement types.