Nora, that's great. Go with it, you don't really have to explain much, I don't really think people are really wanting your life story. What I think they want is just to sort of make you feel comfortable, and you let them know that they are doing that by just being pleasant and proceeding as if you belong there. Takes time, but I think we know when we cross that abyss.
My wife and I were shopping this past weekend. We were getting "can I help you ladies" with no subsequent outing so it was really nice. I'm not in the least pretty, but maybe pleasant looking older chick on a good day. I had my eyes done at the Clinique station at Macy's and my wife was with me and then left to look at some clothes while we were playing with shadow and colors and such. The young girl, 20 something, was really sweet and when my wife left she said, "is that your friend?". I just smiled and answered, "no, actually she's my wife, I'm a trans person. We've been married nearly 40 years". Response was a big smile and no muss, no fuss. She said she thought that was very nice and I swear, she went way out of her way to teach me eye stuff and do anything she could to help.
All nice people want to do is engage, and when they make an effort I think so should we. It's getting easier to just forget the whole trans part of who I am even if I get "sirred" and just be the "me" part. Took awhile, but it'll happen to you too if you let it. Your recent experience is just one of many to come, so be ready. Toni