This is perfect as it is; it might be just a bit more natural to say, “To perform successfully at the highest level, professional boxers need to build up both their physical and mental stamina.”

Yes, just don’t split the phrasal verb here: “If you exercise for a long time, you build up stronger legs and arms.”

Great example Aram. A couple suggestions. We say, “train for a marathon” rather than “practice.” And “for that matter” doesn’t quite fit in this situation. However, you can say, “I once trained for a marathon, too. On the day of the race, I got leg cramps at the two-thirds mark and couldn’t finish the race. I guess I didn’t build up enough strength for that race.”

Fantastic example! Just say, “… by providing great products…” without “the”

Great. I would say, “It’s necessary to get three doses to build up immunity.”