Keep in mind a single entry tourist visa is VALID for 90 days <- meaning you have that long to ENTER Thailand on the visa.
BUT
when you do you will get stamped into the country for only 60 days. <- the amount of time ALL tourist visa holders get stamped in for.
If you want to stay 90 days you would need to go to the immigration office and BUY a 30 day extension for 1900baht.
CHECK YOUR ENTRY STAMP when you're stamped into the country so you know how long your admitted until..
I know this sounds harsh boyz and squirrellz :o BUT it's your passport and your responsibility.;)
The immigration office has heard every excuse in the book about how people end up on overstay and believe me to them, your situation is not 'special' :P
Good Luck
This message brought to you by 'Just-Ask-Tod' <- the guy is an asshole but he knows thingz.