Exersise Advice (planks)?

Ok, im just getting into excersis and its always been in my nature to push myself to extreme limits.. such as i did 50 push ups and no rest then hell did i feel it for 7 days solid... but im now doing planks and its said on advice "10 seconds a day, add 5 seconds each time you feel convident" i did 20 seconds :P but can i do another 20 seconds and more in one day if i feel up to it... or will it damage me?