I train muy thai and i can totally see where your coming from in regards to fitness. Does your gym offer a conditioning class? If so I would definitely take that that will help you alot. If not maybe you can buy a used punching bag and practice your strikes while constantly moving on your toes and practice your kicks, elbows, and knees always moving. Throw your strikes and then move and bounce around like someones countering it will also help develop your footwork. If you rather not buy a punching bag SHADOW BOX SHADOW BOX SHADOW BOX. It's hands down the best thing you can do and it will help your fitness as well as your form.