Even just 5 minutes of planks got me sore arms, abs and thighs!!! You're supposed to do 3 - 5 reps, holding each plank for 20 - 60 seconds. I struggled to hold for 20, but even that was enough for me to feel it the next day.
Did okay on Day 10 (I think I'm a day behind R and GW, did you start May 31, or June 1?) w/ Slim Fast shake for brekky, potatoe soup for lunch, grilled chicken breast for din, no snacks, 2 cokes, but no water and only 5 minutes of planks.