Alice I didn't think about it before, but if you look at your badges you will see you do have the 52 Weeks of Accuracy & 52 Weeks of Photos badges. These indicate you are participating in those challenges.
My understanding of the 13 in 13 is it can be any consecutive 13 weeks that you participated. There isn't an automated tracker of the people participating, you just have to keep track yourself then post an answer to the original question that you have completed 13 weeks in a row.
Lots of people comment instead of answer - it took me a while to get the hang of it when I first started.