help me get smaller!

Before I got with my boyfriend I was super active and exercised religiously and also worked in a labor intensive job. Well since we got together and I got my first ever desk job I've gained about 15 lbs. In 6 months! I can usually put off weight super easily but it's not budging this time. I know the scale doesn't matter but you can tell I look bigger and I can't even run a mile without breaking anymore :( any secret tips or advice to speeding up my metabolism and moving along the process ? I'm just so not used to not being physically active at work. Help!