You can walk down the concourse but it is seriously a very long walk, and I absolutely hate walking it.
It is much faster to take the metrolink and clear security that way.
I honestly do not know if the E terminal would accept your AA
boarding pass, they may just insist you are in the wrong terminal, even though you clearly know what you are doing.
But in terms of time, Park your car at the East Terminal, have him walk to the E then you take the metro over to Main and clear through there.
Just remember when you land back in STL
, you will have to take the metro back to East.
I assume you know it is free to ride the metro between terminals so cost would not be an issue there.