Whether you will find yourself fitter when cruising is directly related to how fit you were before you left. If you were sedentary, you will be more fit cruising. If you were very active on land, you will lose fitness - no two ways about it.
When we lived on land I walked to work
every day, clocking five miles a day on moderately hilly terrain plus all of the walking I did throughout the building to talk to people in person vs picking up the phone
. I also did yoga regularly, and I used to ride my bicycle. 15 miles was a warm up.
I can honestly say that after cruising full-time for 2 1/2 years I am in the worst physical shape of my life. The weight is the same, but I have much less muscle. Swimming, snorkeling, walking around a bit, yoga in the main cabin
, and pulling sheets
while sailing is no comparison to the regular exercise I used to get.