Just my opinion and could be off the mark, but I think there's a stronger than strong possibility that your g/f will know that your lying. She might not have much evidence, but she'd have to be sub-normal not to know it ain't true. 40mins for beer, new area old friend, out of character. I've heard more convincing stories on Jeremy Kyle. I think you're already busted. If she can live with it without further inquisition then it's probably because you're in the last chance saloon. Think you need to be a million times more careful or just don't. I'd say for the next 6 months you need to avoid even a hint of unaccounted time.
It would be interesting to know how old you are. It you're over say 25, me thinks you have a bit of a problem.