Unmarried ladies and gentlemen walk alone in JA quite a bit. At Sotherton Mary and Edmund walk quite alone leaving dear Fanny benched to witness the distress of others as well as mull over her own. In P&P Caroline spirits Darcy away from the others to ramble about Netherfield on narrow walks in Ch. 10 and Lizzy and Darcy walk alone in Rosings Park three times in Ch. 33 much to her confusion. Charles Musgrove asks Captain Wentworth to walk Anne home to Camden Place in Ch. 23 of Persuasion and Catherine and Henry Tilney walk alone to call on the Allens and then back to the parsonage in NA, Ch. 30. It was fine for unmarried ladies and gentlemen to walk about alone as long as they did not do it repeatedly with the same partner—if they did then folks would begin to see it as a preference. Maria is not censured for walking alone with Mr. Crawford but I do think it was it was terribly inconsiderate to Mr. Rushworth. Their engagement also makes her situation delicate compared to the other young ladies present:

Engagements which collapsed at the settlement stage tainted a woman’s reputation, so publicity in the nervous months between the promise and the wedding was a mixed blessing for elite brides. As Hugh Kelly warned in 1767, “of all the stages in a woman’s life…none is more dangerous as the period between her acknowledgement of passion for a man, and the day set apart for her nuptials.”

The Gentleman’s Daughter, Women’s Lives in Georgian England, Chapter 2, Love & Duty by Amanda Vickery

I think Maria feels the opposite of danger in her situation believing there could be:

“no harm in her liking an agreeable man— everybody knew her situation—Mr. Crawford must take care of himself” (5)

Maria’s ideas are quite in line with Henry’s feelings:

“An engaged woman… feels that she may exert all her powers of pleasing without suspicion. All is safe with a lady engaged: no harm can be done” (5).

I am not so sure all is safe with a woman engaged because Maria seemed quite jealous of Julia sitting on the barouche box with Henry. (:D)