Jon Benson is a fitness expert whose books include "Fit Over Forty." He underwent a tremendous physical transformation himself, so he's writing from experience (his site is www.jonbenson.com). He says that one of his most important tips is to wear a heart monitor so that you know exactly what you're doing when you exercise. This isn't a fitness blog, of course, but I thought that advice really translates well to writing, too:
If it's a challenge for you to take the time to write, it's important to set goals for yourself and keep a record of how much time you spend when you're in the research and planning phase, and then how many words you write when you're in the writing phase.
Jot it down on a calendar daily and at the end of each week and month you'll know whether you've reached your writing goals.
We can take another idea from the exercise world: they say that to lose fat, you should exercise at roughly 65-75% of your maximum heart rate. In terms of writing, figure out how much you'd write if everything went exactly to plan. If you hit 65% to 75% of that maximum, you'll do well--because nothing ever goes exactly to plan.