http://www.twohundredsitups.com/
Here is the printable:
http://www.twohundredsitups.com/thsu_bw_pocket.pdf
I find my core gets stronger from pushups so here is that program
http://www.hundredpushups.com/
Week 4 it gets really hard - just keep repeating week 4 if you need to. I was pretty psyched I could do 50 pushups in a row - never made it to 100 but my arms and core were strong.
I am 44 BTW.