STL-ORD is a short flight. If you can't get an earlier flight, I would ask the agent to change your seat, if possible, if the flight isn't full. Sitting in a middle seat on a short flight is do-able if it means you are sitting at the front of the plane and first off to deplane. Agents 'release' seats for those arriving late 10 mins before departure so you can wait for one of those. You can always ask one of the f/a's if they can ask another pax to switch seats to the front. (only tradeable if it is not a middle.)Tell them your situation.
Know in advance your arrival and departure gates. How far... and look at the ORD terminal map in the AA magazine so you know where to go. Be proactive.
Good luck.