By way of planning a lesson mentally instead of writing it down, it is likely that you are planning to fail as you cannot get a concert picture of how events look like when played out in practice. Such an experience is so unjustifiable, particularly for a novice teacher. In any way, doing so is not a good way of planning. To be honest, I may have times where I might not plan in as much detail as I could (particularly when I become more experienced), but this does not mean I am writing my lesson plan in mind. In any case, what is significant is that I did spend a few minute creating a plan outlining how I would be teaching my content, although all of the details are not well written.