That is very normal even if your not overweight. I've been in many exercise classes that skinny women can't keep up. Your body is not used to it. Your heart is a muscle. You have to build it up just like the other muscles. Do what you can do. Slow down when you need to, then pick it back up again. Drink lots of water. The no pain no gain is old school. You do not have to feel pain. If you overdo it and feel pain, then you won't be able to workout for several days and you will get discouraged.

You'll feel good in no time at all. Good Luck!

Race.