If you use an electric sander (like a
handheld Makita) you''ll be there all day. If you do it by hand you''ll be there all weekend.
See if you can borrow/rent a pneumatic rotary sander that takes 6" sanding discs. Your progress will be a LOT faster. Use 80 grit and a gentle touch, just enough to rough up the old
paint. Also a lot quieter and lighter then the electric sanders (your arms will thank you at the end of the day).
Also be sure to check with the yard if you need to have a vacuum attached to the sander, I believe Herrington Harbor N is a "Maryland Clean Marina" and may require it.
Use a 1/8 nap roller to roll it on.