A mantra is a Buddhism idea – the theory being that repeating a word or sound will help aid concentration in meditation. So why not relate this to exercise and try using a mantra when you are training – think of a word, sound or noise that you feel is powerful, strong and even potentially meaningful to you and try using this as your mantra – when you are starting to struggle during an exercise keep repeating this sound in your head over and over again until the exercise is over. But remember mantras are unique to you so the idea is you keep it to yourself and not tell anyone else your mantra.