Based on managing contractors for years now, I think you might be the most honest contractor I've ever heard of. Most of them bill for more hours than they're actually coding. I don't think this is malicious -- it's simply replicating the salaried structure, where goofing off and Facebooking are still part of a work day.
If you feel like you're pretty efficient, I think fixed-fee projects are good for both parties. Hourly tracking is pretty miserable. My clients are usually excited that I offer only fixed-fee engagements.
If you do continue time-based billing, you should try to switch to weekly or monthly instead of hourly. This allows you to build in a little variability.
Somewhat unrelated: someone posted on HN that s/he only codes 4 hours a day at the most, because most people's brains only have a certain amount of cycles to devote to something truly thought-intensive. Many, many people chimed in with their agreement.
That's an anecdote, sure, but there's lots of research to suggest that much more than 30 hours of productivity per week is a fantasy.