If you get a plumber to fix your toilet, he doesn't leave until the job is done (or at least until his part of the job for this day is done). If you go to the dentist, he doesn't let you go until he finishes whatever he decided to do with your teeth.
Organ practice can be like that.
An organist with professional attitude doesn't stop practicing until she has completed her work for this day. She doesn't check her email, turn off her cell phone and any other distractions. Sure, she takes breaks, drinks a glass of water, stretches or takes a walk but later she comes back to practice for more.
In the middle of her practice she doesn't listen to this voice in the back of her head which says: Oh, you've done enough for today, go now and watch some TV. If she decided she will practice for 90 minutes, she will do that for 90 and not 85 minutes. If her plan was to practice for 30 minutes, she will not leave after 28 minutes.
The reason we want to quit practicing early is because of Resistence (Steven Pressfield's term) - this wierd internal force which tries to sabotage our success in whatever it is we decide to do.
Every day is a battle between you and Resistence. You can win this battle by wearing a hat of the professional.