I am quite new to SF as well, and can't say much in regards to yoga, but may I suggest convict conditioning core series? www.scribd.com/doc/31798365/Convict-Conditioning-Charts
Scroll down to "The Big Four Exercises Summary Sheet" and look at the "leg raises" series. Step 1 or 2 might be quite easy for you, but the other ones get a lot harder. There are also a lot of videos on youtube for CC if you are not quite sure how the exercises should be performed.
Also the "bridges" series on page 3 will help with your back and other parts of your core.
Hope that helps!