Hi :)
Methadone's a weird thing. It affects the thyroid in different ways. For most, it slows your metabolism down & folks put on weight. For a few, it speeds it up. When I kicked it @ 47, it sped mine up & I couldn't put weight on for some time. (I'm 5'4" & weigh about 130 now).
Motye is right! Age (hormones) make a difference! :) Walking every day is a good idea. So, is eating lean protein & veggies. I'd cut out simple carbs (wheat & sugar, etc.) & also make sure that you're well-hydrated. But if your looking to tone up quickly, cross-training is probably your best bet.
Enjoy your sister's wedding -- I'm sure you'll look wonderful.
I am 5'2, 128lbs...but am 43....was smaller when younger!!! As i got older i realized that "gift" of eating what we want and not gaining, GOES AWAY!!!! I do yoga 5x's a week but in all honesty, the quickest thing ive ever found was walking...at least 3 miles, at least 4/5 x's a week.