When I was younger I use to focus on speed and pencil mileage when it came to drawing, this was drilled in us at SVA's animation department. The mentality of it was...the more you draw, the more bad drawings you got out of your way before the good ones came along. I never questioned it, because at the time 2d animation was huge and it made total sense.
Years after graduation I came to the realization that pencil mileage, just like any other type of mileage needs focus and direction, otherwise you're just driving in circles and not going anywhere.
Slow down, look at a map, study the options before hitting 90 mph. I learn to slow down and pay attention to what I was doing, I didn't feel a need to fill up a sketchbook every week anymore. My sketchbooks became more of an exploration and study of where my lines could go and what they could form. I guess in short I am saying....pay attention to what you're doing and stop rushing your work, it's important to give it the attention it deserves.