Explanations

FITNESS= Good health or physical condition, especially as the result of exercise and proper nutrition.

FIT= If you are fit, you are in good health or physical condition, especially as the result of exercise and proper nutrition.

STRETCHING= To do some stretching is to stretch. To stretch is to extend your body, or some part of your body, as much as you can. Stretching makes our muscles flexible.

TO BREATHE= To take air in and out of your body through our mouth or nose:  /bri:ð/. The corresponding noun is BREATH /breθ/, which is the air that we get in and out of our body when we are breathing.

I GET OUT OF BREATH= I can't breathe well, I'm too tired. If you get out of breath you can't continue running or doing some other physical exercise because you're too tired and you feel you can't get as much air as you need.

STAMINA= Physical or moral strength to resist or withstand illness, fatigue, or hardship; endurance.

PRONUNCIATION OF CAN / COULD

COULD is pronounced /kʊd/, without L.
CAN is pronounced /kən/ or /kn/ in affirmative sentences (e.g: "I can do it"), but it is pronounced /kæn/ if we want to emphasize it and we stress it. On this video they usually stress CAN and COULD because they want you to pay attention to those words, so CAN uses  the emphatic pronunciation. In real life that wouldn't happen.