How do you get as fit as an athlete? i exercise 45mins a day, do i just keep doing more and more until its hours?

Depends on your goal: If you are body building, may require more. If general fitness, and you are already fit, more or less may be required depending on intensity of your 45 min work-out. 3-5% increase in weights used daily not unreasonable target. So time remains fixed but intensity increase if you want to increase muscle mass. Increasing # reps or distance, will increase stamina without adding mass.