20 weeks is more than enough time, especially since you'll have all summer to train. Me too- yay for summers off, but I'm paying for that break during the entire month of May. Each day from here on out is a struggle to keep the kids interested and "tame." Sorry- I get off on tangents easily.

Back to the topic. Yes, that should be plenty of time. There are many training programs out there to get you into a routine. I love the programs at www.trifuel.com. They are free, and what I used for my first IM distance race, and what I'll use for my second in November. I'm a middle/back of the packer and although my training varies tremendously from those at the front, I do it for fun and not for the win. So I train, compete, and finish with a smile. I think you've got plenty of time to get ready!
Which one are you planning on doing??