I consider myself an athlete, which is ironic because I was not very athletic as a child. I was not very coordinated, fast or strong. However, I found in later life that I have great endurance and excelled in sports such as running, hiking and cycling that require staying power. I now exercise nearly every day, primarily cycling but also a lot of walking, weight lifting and kayaking during the warmer months.
It's all in your frame of mind. You don't have to compete to be an athlete, and you don't have to be among the top performers. However, in my book, you have to exercise or participate in athletic activities on a regular basis, and I fully qualify on that basis.