Things like this are an indicator of how the rest of your relationship will be and what type of person he really is. You're right that the lying is the biggest problem (right now anyway). If he can look you in the eyes and lie to you he can lie to you on the phone while she's underneath him just as easily. Hopefully you have enough self worth and confidence to refuse to put up with that and leave. At 55 I doubt he'll change his ways and become a "good man". I'll bet that he has kept in contact with her for a long long time and this other woman still feels like they have some sort of real relationship. She said that "she hadn't realized that you had become a girlfriend, and you should have known what you were getting yourself into". That is disturbing, because she obviously knew about you before you were officially a girlfriend. There is nothing right about this situation. Get out before you waste any more time.
...one can be sure of nothing until it has already happened...