You don't really need to worry about concentrating per se. As long as you put in the time, you can be as mentally distracted as you *want.* The concentration will eventually just "kick in" after a few days of doing it. I am very distractible by nature, so I know what I am talking about.
Of course, there are ways of removing distractions, like turning off your phone or email while you are working. You can deliberately avoid multi-tasking if that is helpful. What I am saying is that you don't have to worry about the concentration per se. Regularity of working habits is the main remedy, not some internal concentration mojo that must be cultivated on its own terms. In this way concentration is similar to inspiration: neither has to exist before the work happens.