accidently driving in 3rd gear

2009 Nissan Altima been driving in 3rd gear not realizing it just got the car it started to overheat ac got warm and the temp gauge went past halfway point by 2 little lines only time I have drove it 600 miles in last 10 weeks mostly in city within 5 miles of home but atleast 12 times back and forth on highway but around 10-20 minute trips no knocking or leaking I know how and what gear now would I know if I damaged my car? it seems fine now but I am nervous and no budget at the moment