How can I swim a Sub 20 minute in a 1.5K?

Hello all, I'm new here and I have a question that I hope maybe someone could help me with. I've been finding it difficult to improve my swim splits over the longer distances. I've been trying for about 3 years now with minimal improvement. I can swim a sub 30 sec.-50 yards and a sub 1 minute in a 100 yards but cannot hold a fast pace over the longer distances. My best distance splits are: 500yds. = 6:18 (=1:15.7 per 100 yds.) - (pool swim) 1.5K = 22:10 (=1:21 per 100 yds.) - (open water w/wetsuit) 2.4 mile = 1:03:50 (=1:30 per 100 yds.) - (open water w/wetsuit) I've been swimming 3-4x per week about 3,000-4,000 yards per day. Mostly sets like 20x100's w/10-15 sec rest and 10x200's w/20 sec rest and 5x400's w/30 sec rest. My 20x100's are average about 1:17-1:18 per 100 yards, and that's the fastest I can go and still do 20 of them. I swim alone at the pool mostly and I don't lift weights much. I have good form in water and a 2 beat kick. Could it be a strength issue? I use paddles and the pull bouy occasionally. My main goal is to go sub 20 in 1.5K swim in open water, with a wetsuit. What workouts or things should I be doing to do this? Thanks much for your feedback, Terry