I am aware that this post doesn't exactly make me look intelligent, but I'm hoping you can learn from my mistake. :) Good thing I can laugh at myself.
If you have ordered a marathon mommies shirt and if you are planning on ironing on your name, please do not do the following.
1. Set the iron to the hottest setting
2. Forget you turned it on and leave it for 15 minutes
3. Leave the iron on the shirt in the same spot without moving it.
Given these circumstances, you will end up with a shirt like mine.
In my defense, I did practice on another running shirt that I don't care about and things turned out fine. That shirt was black though and if I did burn that one, you couldn't see it. AND I was kind of more concerned about not letting the iron touch the marathon mommies writing because I knew it would have messed it up.
So I do think it's possible to iron on letters, just don't get the iron as hot as it will go and move it around as you iron the letters on.