The core of this is taken from the zero to 1650 yards strategy found at http://ruthkazez.com/swimming/ZeroTo1mile.html
The only reason I'm adding it here is the fear it might one day disappear and be lost forever. Follow the link above if you'd like to see more programs once you get going.
If you have poor technique you'll find this harder than it needs to be. Whilst the above link says to leave technique out of it, I think it would be a wasted opportunity if you don't already have a decent technique. If you can, try and use it as an opportunity to get your stroke right as well as gain some endurance.
Six weeks seems to be the most common length of time it takes to be able to swim 1500 metres without stopping. Experienced swimmers who've just had a break can probably move through these faster or even skip week one.
It requires three times per week and the willingness to be somewhat uncomfortable while stretching your aerobic capability. If we methodically increase our need, our body kindly responds. The amount of discomfort should be small, but it is necessary to pant a bit at the end of each effort and only partially recover before beginning another. The number of breaths taken before continuing I guarantee will not seem enough. I also promise you'll be surprised that you are able to continue much more easily than you imagined. The feeling of not having adequate rest is necessary to improve.
Speed is not your aim during these six weeks - your intention is just to enjoy the water, to relax, or to get some pleasant exercise.
NOTE: Week is 3x the metres. Week 1 is 800 per day, 2400 for the week
WEEK one (Three Days): 4 x 100 or meters...rest for 12 breaths between 100s 4 x 50 metres...rest for 8 breaths between 50s 4 x 50 metres...rest for 4 breaths between 50s total: 800 metres
WEEK two: 200 metres...rest for 12 breaths 4 x 100 metres...rest for 10 breaths between 100s 4 x 50 metres...rest for 6 breaths between 50s, 4 x 25 metres (or 2x 50)...rest for 4 breaths between 25s total: 900 metres
WEEK three: 400 metres...rest for 12 breaths 200 metres...rest for 10 breaths 4 x 100 metres...rest for 8 breaths between 100s 4 x 50 metres...rest for 4 breaths between 50s total: 1200 metres
WEEK four: 600 metres...rest for 10 breaths 300 metres...rest for 8 breaths 4 x 100 metres...rest for 6 breaths between 100s 4 x 50 metres...rest for 4 breaths between 50s total: 1500 metres
WEEK five: 1000 metres...rest for 8 breaths 4 x 100 metres...rest for 4 breaths between 100s 4 x 50 metres...rest for 4 breaths between 50s total: 1600 metres
WEEK six (days 1 and 2): 1200 metres...rest for 6 breaths 3 x 100 metres...rest for 4 breaths between 100s 3 x 50 metres...rest for 4 breaths between 50s
WEEK six (day 3) 1500 metres straight!