Have a great time. The trains do not take that long (take a train to the metro line, sorry, can't remember the lines). You may have to wait a while for the train to come into the station but once on, maybe takes 1/2 an hour???

It won't be a lot of time b/c you'll need to be back at the airport close to 14:00, so 4 hours of sightseeing, not too bad.

Good luck and happy travels!