Good luck on applying for your 60 day visa. Doing this on line is 12 uploads. I just did it and was rejected for an apparent mistake on the date of issue of my passport. This may have been as simple as a date formatting error such as day month year vs month day year? So I'll need to pay again and do it all over. Its possible I will make this same mistake again. It's little wonder Thailand has fallen to 8th place globally for satisfaction for expats, now behind Vietnam.
I had the same question and recently spoke with with a Thai Embassy. They said, their not bothered about the exit date for a visa. However. The airline taking you to Thailand may turn you away at the airport if your itinerary is longer than your visa. It might be advisable to have a flight out before the visa expires... I'm pondering the same situation and nobody knows what will happen at the airport.
Best to drop into immigration and ask exactly what they require. Don't usually need to wait long to ask a question and it seems not all offices ask for exactly the same things.