Question 055: Fitness for TKD

I'm looking for fitness advice as I'm training for my black belt. Being over forty, and entering my fifth year, I have improved my level of fitness but it is not comparable to others. I have not been an athlete, nor have I stuck with any exercise program or sport until I tried Taekwondo. So, needless to say, my level of fitness lags compared to others. What advice can you give?