Work from your calendar, not a to-do list.
Most successful people don’t use to-do lists to organize themselves; instead, they put everything on a calendar. If something is really important, it should have a slot on your calendar.
Treat the blocked time slots as if they are doctor’s appointments. They are that important. Following your calendar is more important than an unplanned talk with a colleague. Don’t cancel goals. Reschedule them if necessary.