I was diagnosed with marginal placenta previa at 12 weeks pregnant after some pretty scary bleeding and cramping - placenta previa is very VERY common, and very rarely causes complications. Unless you have a history of bleeding and miscarriage in your past pregnancy, then the doctor will just keep an eye on it, and allow you to go on with all of your normal activities until delivery. A great majority of pregnant women have marginal placenta that never needs to be addressed because it causes no problems - and the more marginal, the less serious.
I had no complications while carrying my child, and the birth was perfect and went as smoothly as anyone could have hoped. I worked a very physical and stressful job up until the day I delivered without a single complication.
Don't be scared, and don't be worried. It is very unlikely that the marginal placenta, even if it turns into previa, will have any bearing on the way you carry or deliver your child. Just remember that it is common, and if your doctor doesn't seem worried, then you are doing just fine.