Marcia,
Here's my story--I am a cardio junkie as well. I used to run marathons. Although I only did six, I've run lots of other distances as well. After I set a personal record in a marathon, I tried to run the following week. I realized that I was injured with ITB (Illiotibal Band Syndrome). I couldn't run and it felt like I lost my identity. Then I discovered yoga. I started doing power yoga. Although it's not really resistance training, I was able to start sculpting my muscles. Then I discovered weight training. It was only after I started lifting weights did I actually see the changes in my body. Now I'm a yoga instructor and personal trainer (moonlighting). I've never been more fit. I still do cardio about 30-45 minutes per day, but once I started lifting heavy weights (!) is when I noticed the change in my body. Cardio strips the fat, weight training sculpts the muscles. I think that clean eating is 70-80% of the equation. This means that I can literally work my butt off in the gym, but negate everything that I've done with one meal. I eat to fuel my workouts and sculpt my muscle.
So, if it takes a while and you're will do go the circuit route. However, I promise you that if you make the commitment to lift weights (heavy for you) then you will really see the changes in your physique.
Take care,
Laura
"The difficult periods of life provide the best opportunities to gain useful experiences and develop inner strength. " ~ Dalai Lama