At a normal kind of job at work, you wouldn't make excuses for not doing your job. You might - people do - but it doesn't exonerate you from getting the work done.
As writers, if we consider this to be our work, we shouldn't accept excuses from ourselves. We should set reasonable plans and expectations given the number of hours we have to work. And we should hold ourselves accountable. (Yes, this is a pep talk to myself.)