Pregnant and overweight?

So I recently had my first prenatal appointment and the baby is great and she said everything looks good. But then she said that for this pregnancy our goal is to only gain 15 pounds because I'm already overweight. I know I'm not in the best shape and of course I don't want to gain like 80 pounds but I'm kinda worried now like I feel I should have lost some weight before I got pregnant, which I was trying to but at the same time ttc. It's been 6 months trying and kind of took us by surprise. I'm 8 weeks now and 137 lbs, any advice on how not to gain to much weight since I have 7 more months to go?

***update***

I'm very thankful for everyone's positive comments and advice. I'm not scared to gain weight I knew that I was as soon as I found out that I was pregnant. But since the doctor seemed concern when she told me I got a little worried too. Of course it took me by surprise when she said overweight because to be honest this is the sexiest I felt in a long time could be because I feel like I'm glowing but not letting anyone burst my bubble lol anyway, I am eating healthier and walking some I just want to be the healthiest I can be for my baby. Thanks again!