There is something to be said for getting into the flow state. I’ve often written for hours on end. But I’ve also noticed my attention can begin to waiver and often requires more effort the longer I do the same task.
Definitely do what works best for you. If it ain’t broke, don’t fix it. But my guess is that most people struggle more with getting work complete as the day progresses.
This is an effective research based approach to help those who struggle with wavering focus throughout the day.
Getting into the flow state and extended periods of deep work is a more advanced strategy that takes time to develop. I do think people can extend those times, but it requires a solid foundation with persistent habits. You have to get more familiar with what works best for you and track your progress along the way.