From Alice in Wonderland, by Lewis Carroll:“There is no use trying; one can't believe impossible things." -Alice
"I dare say you haven't had much practice. When I was your age, I always did it for half an hour a day. Why, sometimes I've believed as many as six impossible things before breakfast.” -Queen
On one hand, faith is about believing beforehand; presupposition that something is true. On the same hand, the less evidence one has, the more virtuous the faith becomes, and the more proud the person can be for staying faithful to the belief in spite of the lack of evidence.
On the other hand, you have utterances like the one you quoted, about having evidence for faith. "It's not like there was no evidence; after all, Mary knew of a woman who hadn't had babies and therefore was known to be 'barren', but was now suddenly pregnant." But can it really be said that being childless/barren and becoming pregnant is on the same footing as getting pregnant on your own? Why does the Watchtower even mention Elizabeth's circumstances as proof, by the way, when Mary was visited by an angel, which would be much more powerful evidence of something supernatural taking place?
What Luke really says is "God did it.". That explains everything. I never had a problem with Mary's miraculous conception, 'cause I believed 'God did it.', and he can do anything and has created everything. No biggie for him. When you have that presupposition at the base, anything thereafter can be rationalized. You can easily believe impossible things six or more times before breakfast, because God makes it possible.