Billing Hours

I am at work, and I am writing my blog. Terrible! I fell guilty.
I am payed a fixed salary, but my bosses bill me out by the hour. It makes sense for our company and for the client. But not for me.
I am supposed to bill 1800 hrs a year. I am far from my goal this year. However, I have never turned down an assignment and have never been even remotely close to late in turning in a project! I work overtime and on weekends when necessary. My main problem: I work too fast. Other people may be able to bill 12 hours on something that will take me 8. So, I end up with a lot of free time, when our office is not swamped with work. I do not feel right making work up for myself and billing it to the client (although probably nobody would complain). I try to do the activities that are really helpful to the project, or those that are assigned to me even though I don't agree.
I've been told to pad my hours (not by the bosses). I don't feel comfortable doing that. However, If I bill less, my bosses make less money, and I work for them. I am supposed to make them money. If I had enough work all the time I would't have this problem, but work tends to come in spurts, and there are many days when I can't do anything billable.
Right now the boss is in a meeting with the client and has all the documents with him. So, I can't work on this project. I have no assignments on any other projects... what am I supposed to do???