The beauty of innocence

My 3 year old son went with me to the gym last Thursday. As we were leaving he asked me why I went to the gym. Not wanting him to see my weight or "being fat" in a negative light I told him the following: I go to the gym to get stronger. I go there to build strong muscles and to be healthy. I told him that he would be riding a bike very soon and I wanted to keep up with him.
He answers me from the backseat with "but Mama, you have big muscles. You are strong. You can pick me up."
I then explained to him that very soon he would be riding a bike and I know that he will be very fast. I explained that I wanted to be able to keep up with him so that he wouldn't have to slow down because of me.
He's had my butt in the gym almost every day since then and reminds me constantly why I am doing this and why I am here.
I think I am awesome, my son thinks I am beautiful, and my husband lets me know every day that he loves me. Why would I not take care of myself and grow old with them?