Congratulations!!!! What wonderful news.
But I agree, this is no time to abandon good eating practices. None whatsoever. Or exercise for that matter.
They say that pregnant woman, starting in their second trimester should add only 300 calories extra per day and that's only if they're a healthy normal weight. Those that are overweight are advised to not gain anything. The old wives tales that you're eating for two is absolutely false. 300 calories of additional healthy foods - that's it and again starting in the second trimester.
As far as exercise, you can continue with whatever you were doing for the most part. And yes walking, swimming, even strength training is very beneficial. Speak with your doc.
Enjoy this time, it's a wonderful experience.