@afewbugs For me it takes mental effort to initially direct my attention onto the important task, but once on-task I can focus for about four hours without much additional effort. Then I need a break.
The big difference between me and my slightly ADHD husband is that I can tune out background noises and he can’t. For example, on the subway there are repetitive announcements (like “please hold on while the train is moving”). I ignore them, but he is completely distracted by them every single time they play.