OK, I'm going to make myself unpopular here by saying that I can see where you're coming from.
DISCLAIMER : I still think that the way you went about it, and your general understanding of how grow ups should behave, sucks. You seem to think that your boss is somehow like a teacher, and "telling on you" is against the rules of the playground.
BUT
It sounds like the task of manually inputting this data is a tedious one, which was historically done by the person in your colleague's role. It isn't your data and it isn't hers - it's just that someone needs to do the boring data entry part.
If you don't think it should be you, you're going to have to come up with a better reason than "it's easier for me this way". That makes you sound, well, lazy and childish. Go to your boss with actual reasons, such as :
"Data entry is always a boring job - in my role I take on xyz [ list parts of your job when you do something tedious for the good of the team] and historically, updating this spreadsheet was part of [colleague's] role. If you'd like me to start taking responsibility for this spreadsheet as well, that's no problem, but it may mean taking time away from [list an area of your role that you don't enjoy and would be happy to have someone else take over]. How would you like to proceed with this? "
Of course, if you frequently" sneak out early " it will be harder to claim that you don't have time to do this particular task.