I kept up with one letter per day through the 11th, but then I missed a day and had a huge emergency that took more than 24 hours to deal with. So I mailed 4 letters on the 14th to catch up. Sadly, I failed to actually mail anything on three days the USPS is open (I started out doing the LetterMo rules, and added InCoWriMo later), but I have mailed 15 letters in 14 days.
Since I added InCoWriMo, I have used harvested addresses from their website for the extra (Sunday and President's day) letters instead of people I already know. If anyone wants an InCoWriMo letter from me, the first three to PM me an address win (two Sundays and President's day still to go).
Hopefully I can keep it up for the rest of the month more like the first 10 days, without missing days.