Today it was 240 very LONG seconds.
At 2 minutes left it felt very long to the end.
When it was 1 minute left my legs were starting to shake, maybe it is not such a good idea to do the plank challenge after a 10k run. The legs started to shake more and more. I was wondering if I would be forced to put down my knees on the the floor but tried my best to not make that happen.
18 seconds left, let it be soon over
4 seconds left and finally I heard the beep. 
