Cheri,
i'm one of those skinny gals too.
It took me a few years to build the strength in my legs to where i could ride more than 10 miles without feeling lifeless.
I would say to my husband, i'm all pooped out, i have no poop.. .
I didn't need food, water... I simply needed to build muscle. and it took a long time. I'm still not fast, but i can go forever now.
Hills will really kick you, but just get down in your granny gear and keep pedalling. I never get off the bike unless i am breathing too hard. (I'm 54, after all)