Robert Louis Stevenson offered that advice. It's good to remember on those days, weeks, or months when you feel frustrated.
Maybe the words--or at least the right words--aren't coming fast enough.
Maybe agents or publishers are making you wait forever to hear whether they are interested.
Maybe that self-published book of yours doesn't seem to be catching fire despite the fact that you're working hard to market it.
Stay with it. Looking forward, everything seems slow. Looking back, it seems as though it all went by in an instant.
Set yourself a schedule that's ambitious but realistic and stick with it.
On days you begin to doubt whether you're on the right track, keep going.
If that keeps up too long, take a little break, but schedule the resumption of your writing on your calendar.
Even if the "planting" doesn't pay off in a trip up the best-seller list you may find that you've gained in ways you never imagined.
(If you'd like to learn to write from the best, get a copy of my new book, "Your Creative Writing Masterclass." It features writing advice from more than 100 classic and modern writers, plus my guidance on how to apply that advice to you want to write. It's published by Nicholas Brealey and you can get it from Amazon or your other favorite book seller.)