Week 4, Day 1: Two Hundred Sit-Ups

It's getting a little tougher now that I am on Week 4, Day 1of The Two Hundred Sit-Ups Challenge this morning.  I am still able to do 50 sit-ups in set 5.  However, the burn is starting a lot earlier - like in set 2 instead of set 4.  My abs are getting tighter although I can not see any physical difference.