Advise needed to improve swim times

At 3:00/100 there are technique issues that are holding you back.  The best thing you can do is get with a qualified coach/instructor that can help you with your technique.  While you can swim 1,500m, you don't really have swim fitness because your technique is flawed.  Technique and swim fitness are two sides of the same coin and you can't have one without the other.  Good swim fitness is built on good technique and good technique is built on good swim fitness.

If you can't find a coach/instructor locally, there are numerous resources on-line that can help you.  The problem with watching videos and reading articles is you won't be completely sure if you are "doing it right."  Seeking a video analysis can at least get you some feedback that will be helpful.  You can read this article on my website that talks about the fundamentals of swimming.  Most adult-onset swimmers overlooked the fundamentals when they began learning to swim.  Those fundamentals - blowing bubbles in the water, learning to float, developing comfort in the water, kicking from the hip - often are at the root of many technique issues later in their swim careers.  Beyond that, given the pace at which you are swimming, I can say with almost certainty that you need to improve your position in the water - your balance (this goes back to the fundamental of "floating").  Your legs are undoubtedly dropping which is introducing drag that it's impossible to overcome.

I can't overstate the value of a qualified coach or instructor.  There is no replacement for "eyes on deck."  The instant feedback from a coach on deck is invaluable when learning proper technique.  Absent a coach on deck, a combination of self-instruction and remote video analysis will get you swimming properly, however it will take a longer period of time.

Next thing, once you begin to improve your technique, you don't need to be doing long swims in your workouts.  When you swim long continuous swims - i.e. 1,500m - you fatigue.  As you fatigue, your technique breaks down.  Every stroke you take is being stored in muscle memory.  Ideally, you want to be storing proper technique in muscle memory.  By doing repetitions of shorter distances with rest in between, you can maintain proper technique longer (assuming you have decent technique to begin with).  For example, instead of 1,500, maybe 15 x 100 with 15-20 seconds rest between repetitions.  

Good luck, hope that helps.

Lessons!  

As you've said, you're a runner for the last 7 years so cardio fitness isn't the problem.  

Given that you're a runner, it's probable that you're body fat is relatively low (possibly despite your username) and leg density relatively high.  You're likely to experience legs dropping even when trying to hold proper position and this gets worse if you lift your head even a little bit.  Coached training for even a few weeks gives you some immediate feedback, drills, to improve your technique.  

When you swim 1500 meters, how quickly do you feel your form degrades and you slow down?  50m?  100m?  300m?  

Breaking your swim into shorter sets (50-200m) with enough rest that you can do 8x200 where the first, second, seventh, and eighth 200 are within 2-3 seconds of one another is a good start.  

Assuming you can hold form for 200m (if not, use 100m), time yourself for 3x200 with 1 minute rest between.  Now do at least 1x200 using a pull buoy at your knees and a 1x200 with the pull buoy at your crotch.  If you're faster using a pull buoy and not kicking than swimming while kicking, body position is a key contributor (kicking efficiency a strong consideration as well).  You might borrow a pull buoy from the pool.