I'm relatively new to SUP paddling, and I went out today to see what I could do. Sadly, my GPS batteries gave out before I could even get started.
Anyway, I paddled my usual lake which is about 2.75 to 2.80 miles around depending on how close you follow the shoreline. It took me between 40 and 45 minutes (guessing on my analog watch, should have used my stopwatch). That puts me somewhere between 3.5 and 4 miles an hour.
Touring Kayak Paddles
URCHIN Portable Anchor