Whatever your personal intentions are about making such changes, the key is to commit. This means physically signing up now, today. Don't wait until January 1 when the urge, while nice, could quickly pass.
We all make New Year's resolutions at the same time, but there is no reason to think we are all comparably ready to make lasting behavior change at the same time.
The more we age, the more hormones we lose. Without the right levels of hormones, many women, and men as well, find that they just can't lose weight, no matter how hard they try.