So you didn't train too hard and kept it all easy. You sound as if this would be desirable. However, maybe that was the real problem. To improve you have to put a certain stress on your body and leave your comfort zone repeatedly, so the body feels a need to adapt and increase its performance level.
Regarding the other girls beating you while doing less. Maybe they are more talented, but I don't think so. A lot of people are training their arse off but would never admit it. It is always better to be admired as the effortless, talented runner than the poor hard working one who runs much slower than average for a certain mileage.
So a lot of these '20 mile / 4 times a week' runners are training almost every day and sometimes twice in reality (resulting in 50 and more miles occassionally) but just don't tell it.