Would you feel better if you drove it more? If so you should take it to the grocery store and the post office and just use it for daily errands. Start taking your friends to the airport... in other cities.
I'm not complaining, far from it. I'm glad I have it. Mumsywumsy likes it, she even volunteered, that she had fun on the last trip. I will start using it more, but at the cost of fuel there's only so much I can spend, so building takes precedence over using right now. On the spreadsheet fuel cost per mile isn't likely to change much, but as the miles increase cost per mile figures will go down. Not that it really makes any difference.
I like to play with spread sheets too, I had one on the semi that told me exactly how much it cost me, by mile,day, month, and year to operate, both loaded and unloaded.