One thing that has struck me on this latest re-read is Marriane's assumption that the Colonel is physically incapable of love due to his age.

" she was reasonable enough to allow that a man of five and thirty might well have outlived all acuteness of feeling and every exquisite power of enjoyment."
Thus on first meeting him he his discounted as a suitor purely on an age basis. I think it's true that to most teenagers think that anyone past their mid-twenties is old. I certainly wouldn't encourage my niece of 17 to marry a man of five and thirty. But such a disparity in age was acceptable in Jane Austen time. Marriane therefore is acting on her own youthful romantic notions-she has conjured up in her mind what the perfect man is, and unfortunately the Colonel doesn't fit the bill.