Just come out and tell them.
No harm done really... it's not like lying about boxing is that important. And at least then you can feel good about coming clean. Maybe they'll think it's funny. And if you let them know that you're not to be trusted when you're drunk, you'll cover any future drunk lies, too.
I think the lying is closely linked to the alcohol- many of us seem to have come up with a few. Best to avoid the alcohol and then you don't have to worry about it in the future.