I keep getting swimmers telling me that they 'did a 5K today' with fins and sometimes paddles, now what is wrong with this picture?
If I entered a half marathon would they let me ride my bike? It is the same difference except that by using paddles that much you are just asking for a shoulder injury. The use of fins and paddles are usually a treat, not a staple of swimming. Paddles are used just to figure where you are slipping your hand and then a swimmer goes back to no paddles as not to lose the feel of the water and not to injure the shoulder.
Now I do see new swimmers using fins if they can't make the interval in the slowest lane for a short period of time but to do entire workouts with them forever has no benefit. Short fins such as zoomers are useful in building the legs during kicking sets.
If you can't do a 5K without fins, then maybe a good 1K or 1.5K is a good place to start and build up from there, you will get to that 5K mark and it will actually mean something.