45 minutes of sprints would probably be too much. And sprinting every day is too much. Mark recommends every 7 to 10 days and doing something like 8 sprints with some rest in between. His sprints take me about 15 minutes. Alternatively you could try Tabata sprints which take you only 4 minutes to do and are suggested to be done something like twice a week. Four minutes of Tabata sprints IS a workout and it sucks royally. Try it sometime and you'll see how much it sucks.
Originally Posted by Miguelinileugim
Brief high heart-rate sprints are good for the heart. It's that marathon training that causes the enlargement of the heart that is not healthy. Think of it this way: low weight, high reps causes muscle hypertrophy while high weight, low reps builds strength. Your heart is a muscle. Marathon training is the low eight, high reps and sprinting is the high weight, low reps.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Heaviest squat: 180 x 2. Heaviest Deadlift: 230 x 2