One of my big motivations is that I know there are a lot of swimmers, many of them in senior age groups that can swim 25km at pace splits of 1.25/100m or under.
I do not believe they are either mega human beings or infinitely fitter than me, they have just taught themselves to swim easier and more efficiently.
I would set your goals as high as you dare. Personally, I think a sub 24 minute mile is within most peoples ability, even though I am not there yet myself (28.08)
Its also encouraging to look at the 1500m masters world records and see just how fast the 70+ age group swimmers are still going.
Once you accept how effortlessly some people swim it becomes a question of solving the easy swim puzzle rather than a challenge of physical endurance or fitness.
If you are already at a 2.02 pace since starting in May then you will make the speeds mentioned above no problem, 1% of efficiency at a time.
enjoy the water.