I probably did a bit of a disservice by focusing so heavily on time. Simply committing to putting in the hours is a good motivator when you're looking to get the ball rolling, but it shouldn't be used as a singular measure of success.
I've had plenty of days in which I've barely done half of my 'quota'. That could be seen as a failure when looked at in isolation. But when widening my perspective and considering what I've actually achieved, I'm generally satisfied. And really, it's only when you're not satisfied with what you're achieving that you need to question whether you're trying hard enough.
It goes back to what kb and veen said, in that you need to recognise your victories and successes as well. If don't, you'll risk focusing on the wrong things and burning out.
I like what Leonard Cohen said:
[Writing] begins with an appetite to discover my self-respect. To redeem the day. So the day does not go down in debt. It begins with that kind of appetite.
That's what I think about. Have I achieved something today? Have I redeemed it from being wasted? If I have, great! Keep grinding. If I haven't, what can be done to change that?
Putting in more time is the usually the answer to that question. And that action will be accompanied by victories to recognise.