Surgery booked but I will hide It to girlfriend

[FinallyBionic]

Oldboy,
Telling about an implant before you have it may not sound good for many women. Before knowing about the implant, I wouldn't accept even thinking about it. Only after coming to FT and reading a lot of posts, I was really convinced to do it. Your girlfriend might not accept the idea and try to discourage you. This is just an assumption, but it happened with me and others.
On the other side, since everyone does it because of ED, she already knows that you have ED, even if she didn't tell you. When women love, they don't want to hurt the feeling of the partners (men as well). She will know from the first time you have sex with her, and as most of replies here told you, it won't take a day or two to be discovered.
My suggestion is telling her that you are going to tell her about some positive change to your relationship very soon, and that you won't be able to tell her now, then mention the hernia or other similar surgery. That way, when she knows about it, you will tell that is what I talked to you about before, and that you were not feeling good about making her satisfied with sex .. etc