Did that - Dublin marathon last year. finished the 26.2 jumped straight on the bus back to the airport, flew back to Gatwick, train to Victoria, stood in the tube the whole way home.
I guess I survived, but I don't recommend it - so my sympathies are with you - best to try and keep moving around as much as possible, especially on that plane flight, and really dose yourself with fluids (not all of the alcoholic variety either). At least you're not going into work the following morning! best of luck with the 1/2M. s.