Hi - good luck in that career choice! It's a wonderful opportunity to really make a difference in people's lives and improve their overall well being.
There is a real “people side’ to being a good physical trainer. It’s more than just the science and education of fitness although that is certainly an important part of it. Being able to quickly assess where your client really needs help or to discover how to motivate him or her could be a real difference in how successful you are with a wide variety of people you will work with. It’s easy to relate to someone who is similar to you, but when you are physically fit, and motivated yourself and someone comes to you for help who isn’t either of those things it can be hard to identify with them and find how to best help them achieve their goals.
So the science and physical knowledge of how the body works and how food plays a role is just the first step. Understanding the different psyches of the people you will come across is a very different aspect.