I'm a little fat. I'm afraid she'll see a joke when I run. What should I do?

This is just your personal subjective opinion, not the fact you see.

What are you afraid of? Even if I see it, what can I do? Everyone is unique, as long as you are honest and kind. Being able to run means trying to change yourself in order to lose weight. That's good, as long as you can persist, you can do it.