I knew a Longmaid... and she was always torn between loving it, and hating it!
A double surname (as in, Perfect-Mayfield, or Mayfield-Perfect, hyphanated or not...) isn't necessarily the trouble that a lot of people seem to think that it is. My DC have a double surname. My DD (20) uses the full surname, thinks it's great, especially when coupled with her unusually spelled first name (which is, actually, the proper spelling from a country of her genetic origins!). My son (12), meanwhile, has dropped half of it and goes by my name. Legally, both surnames have to be acknowledged by/for my son on documents (passport, drivers license when he's old enough, etc.), but it's his choice to use the name he prefers.
My name, incidentally, is not my maiden name, nor my married name. I loathed my maiden name, and have never married. I simply changed my surname by deed poll when I was old enough to do so. The fact that I chose my maternal great-grandparents name is twofold - they were fantastic people and I wanted to honour them, and also... my mother was born "out of wedlock" (as my grandmother always politely referred to it) and had her mother's maiden name as a surname. When her mother married, my mother's name was changed - from one which she loved, which had helped to form her identity, which bonded her with the people who adored her... to one which she hated, and felt bound her to a stepfather whom she loathed (although he was a fantastic grandfather, I understand that he wasn't as ready for a petulant seven year old version of my mother throwing him into parenthood as he thought he was!).
My advice, OP, is that you double surname your baby. That way, whether her father and you marry or not, the choice as to which surname she prefers is up to her and she has the option of either using her father's, yours, or both (or changing it by deed poll at some stage!). My DS was 4 when he dropped his father's surname. He was very firm about it, and still is. But he still has the option of using it as and when he feels like doing so, because... legally it's part of his surname. Surely it's better, and less hassle, for your baby to have the freedom of choice over her name when she's old enough to understand how they shape and identify us?