I was in a similar position - if I went back to work after 12 months I would have to do f/t, they couldn't offer me a p/t position. I took the 12 months leave to keep my options open - I was being cautious, just incase of financial catastrophes, or something happening to DH's job etc. As it happened, I resigned 9 months into my mat leave as by then I knew I didn't want to go back.

I think you are well within your rights to do this - like sushee said, no one should expect you to know what you want do at this stage. It doesn't hurt to keep your options open.