According to the Google distance calculator, it only takes 1 hr 16 mins to walk from Southward Park to Westminister Abbey. However, the official British Lying-in-state queue application indicates that it will take 14 hours.
From BBC News, the queue is moving fast. Besides, the application is used to show the actual distance in miles.
What a long queue!!
https://youtu.be/lCHZtDiVgOw
Queen Elizabeth II: Huge queues form for lying-in-state - BBC News