I am 6 weeks, I was running about 3 to 6 miles before I became pregnant and i am still doing it. I am super tired so I don't run as often, and I just eat a balanced diet. Whole grains, fruits, veggies, I don't do too much dairy, I like almond milk.
Walking is a great exercise. Myfitnesspal.com can be a great place to track your caloriea. Make sure you are eating fats but the right kinds of fats. If you can eat fruits and veggies, aim for a few servings a day.