I do this every day. A few weeds pulled here as I walk to my car, a few there as I am taking out the garbage, a few more as I walk to the street to get the mail. Over the course of a week I have accomplished an hour and a half--maybe even two!--in holding back the ever present encroachment of weeds. At week's end, I am always pleasantly surprised by how effective my "drive-by" gardening efforts are: I have only an hour and a half of more serious taming to do, rather than a full three hours of weeding taking up a whole afternoon on a precious weekend.
This strategy works well in many other situations, too. Writing a book, dusting your window sills, and nurturing a relationship.
Small but frequent efforts result in a mostly weeded garden, paragraphs adding up to a chapter, and a mostly dusted house. A smile, a hug, a compliment, or a kind gesture can nourish and sustain a relationship between longer and more intensive periods of focused attention.
Give yourself permission to know that less is sometimes enough when you don't have the energy or the time to do more.