Hey Amanda!
I also had the Mirena but for almost 3 years before having it removed after getting married. I had mine removed on April 5 2007 and had a VERY light period that started the next day and only lasted one more day. Using the counting method to concieve, I just started at the day I started to bleed and had some idea of my ovulation. I was right on the nose because I am now 25 weeks pregnant and she measures perfectly with my math skills.
Like you, I had light periods and lots of spotting for like the first 6 months. After that nothing for 2 years. No kidding, my monthly went away completely. So I worried about my cycle getting back on track. Turns out it already was on track because I was still ovulating the entire time.
My OB at the time gave me a hard time about needing to wait to prepare my body. Something about waiting for 3 months to let my cycles get started like normal. After getting a second opinion on that one ( nothing to worry about, my body is fine) I got a new ob. He told me that everyone has different ideas about how long to wait to concieve after stopping ANY form of birth control.
I hope this helps you out!