Yeah, I know, I’ve already written about the post before. But that was the Persia version. This is the more familiar U.S. of A. =)
In researching how long it might take someone to travel by horse from Williamsburg to Annapolis in 1783–in December–I came across some information about the post at the time. And, therefore, the post roads and regulations. 
Philadelphia was the main Post Office, the kind of central workings. From there branched roads north and roads south. In the southwardly directly, the next major stop was (thankfully) Annapolis. From Annapolis things either went over water by ferry or by land to (doo doo doo DOOOOO) Williamsburg. (Thank you, Lord!!!)
Now, the post roads were about the only dependable roads in the country at that point. They had to be–see, there was a rule that a postal rider must cover 100 miles in 24 hours. In those days, that was difficult. Riders often had to go straight through the night to make their deadline. And the pay, you wonder? For the six good-weather months of the year, April through October, they made 8 pence per mile. In October through April, they made 12.
Why was this helpful for me, you wonder? Because it is oh-so-important to realize that she was traveling solely by post road, which means quicker travel. But even they were tougher to get through in winter–another source I found said to take a good-weather estimate on travel and double it for winter travel estimates.
And there we have a lovely formula to figure out the timing necessary for my story. Lark’s letter would make it from Annapolis to Williamsburg in two days. But it would take about a week for her to travel it by carriage, assuming some rain and mud and cold.
Boy am I glad to have figured THAT out!