OP, if it makes you feel better, I gained 13 lbs in 4 months after we moved from FFX to ARL when I was 39. I have absolutely no idea where the weight came from - I was exercising daily (often at high intensity), walking a lot every day, not eating my feelings, not really eating differently - but 13 lbs just came on out of nowhere. (I'm typically about 140-145 lbs, but ended up at 158.)
I chalked it up to the dreaded Age 40 Metabolism Slowdown and tried not to worry about it too much. I continued to exercise daily, started keeping an eye on my diet again, and have gradually lost most of the weight I put on.
Anyway, forget about "quickly" and just figure out an exercise routine that works for you, along with healthy eating habits. You could try journaling your calories so you can see what you're really eating - for me that's always been a great way to make sense of the number on the scale. ("You mean I *can't* eat baskin robbins every day and lose weight? go figure!")
