Kind of depends on how much you paid for the warranty.
Also, how much is left on the original warranty. Is that just 36000 miles total for the original warranty?
So, if it's 36000 miles....you'll basically have it for a year before that warranty would have expired. Since i'm going to say that most people on average use their cars 15000 miles per year. (36000 - 21000)
so beyond that.....this extended warranty would be for 1.5 to 2 years extra of warranty. Since this is a 60000 miles and the original warranty already went for 36000. Which means that those 24000 miles would probably be used in 1.5 to 2 years (12-15000 a year)
So you'd have to weigh how much you paid for the extra warranty against how long it would last.
If you have any idea of the maintenance that was performed by the original owner, that would be good to know.
I'm not sure how easy this is to do....but.... you might see if you can get a print out of what warranty work was done in the original 21000 miles to see if there were a lot of problems with the car in the past.
You can also go to
http://www.nhtsa.dot.gov/ and look up the Technical Service Bulletins and consumer complaints for your car. That way you can see if you've got some problems right now that might not have been fixed.
hope that helps