I missed a few flights doing this but pre-9/11 it was definitely possible (but not advisable) to get to a smaller airport 30-45 minutes before departure and make the flight.
On the arrival side, back in the good old days (mid to late 90's?) with no checked luggage I could get from the arrival gate to Hertz #1 club gold rental car pickup (when it was in the terminal A garage) at SJC without breaking stride - probably 5 minutes max from the aircraft door to car door.