first trimester workouts

A few days ago I found out Im pregnant, yay! I'm five weeks today, and before getting pregnant I've worked out religiously for years. Like, heavy weights, heavy squats,  & long distance running, those types of things. Well my dr says it's safe to continue those types until I'm 20 weeks?!? That seems crazy to me...I don't want to risk miscarriage or anything like that because I'm continuing to lift heavy! Does anyone have other recommendations on working out while pregnant? Is it really fine to continue to lift heavy and run long distance? Help!!