I don't struggle with this issue of feeling like I could do more with exercise, but I do often feel I could be better in other areas in my life.
With exercise in particular, I find I do have to change it up every few weeks, because my body gets used to the routine and it becomes less challenging. So, maybe switching up your routine could make you feel challenged by your exercise again. Some people also find that having a training goal (like a 5K or a half-marathon) can help as well.
I guess I don't have this problem with exercise because I don't really like exercising. So, I do what the experts say is good for my body, and not much more.
About the niggling feeling of inadequacy, I feel it about other things. I work on accepting both my strengths and my weaknesses, and also reality-checking with people I trust. It's an ongoing effort!
My Sparkpage is set to private, but I'm open to SparkMail!
"People often say that motivation doesn't last. Well, neither does bathing--that's why we recommend it daily." - Zig Ziglar