GOOD for you for getting out there! I can relate to the "if it feels GOOD then why do I just not DO it???" and "I know it is good for me and I feel better when I do it, but I just DON'T and I don't know why..."
Don't beat yourself up for this. I think you've just hit on the thought process that everyone has. It might have to do with feelings of guilt -- as in, I shouldn't spend time doing what makes me feel good when I could spend that time with the kids/husband/job/niece/relative -- it might have to do with feelings of fear about who you really are -- if I love to exercise and I never did before, then who am I REALLY -- it might have to do with wrestling with our own definitions of self -- I've always been the heavy/large girl and if I change so will everthing else around me and if if this what I "am" then how can I change...
Ramblings, I know, but this is where the work is done behind the eating and exercise plan...and I'm not saying you wrestle with ANY of these, just that it is normal IMHO to have these questions. And THAT is scary, and it is sometimes so much EASIER to crawl back into our self-defined box and live there like we always have.
As the Italians say, "COURAGIO!!!"
I'm rooting for you!
Kira
But when I am at my happy weight I like to tell myself that is how I look 
(sadly just not as firm!)