I hate to tell you this, but as a "speed" oriented 1500 whose PR is "about" 4:00, you might have a difficult time breaking 15:00.
Be that as it may, you are going to want to do a lot of over distance tempo work, IMO. The hardest thing for 1500 guys in the 5K (as a former 1500 guy myself) is after the half way point. Doing interval work, even longer intervals like 5x1600, does not prepare you for that. 1500 guys can do 5x1600 at race pace no problem (normally). Better for you is 4-6 mile tempo runs at 5:20.
You could also do something like 2x4000 where the first 2000-2400 is race pace, then the last 1600 is slower as if you were going to keep going to 4 miles.
On top of that, you do need to do the 5x1000, 4x1600, and the shorter 400 and 200 stuff as well. You still need to be able to run relaxed at a fast pace.
Of course, first of all you need to get back to decent shape to begin with. If 70 is all you can do, then you need to build up to that and hang there for 4-6 weeks. However, 80-90 would be better - if you really want to improve enough to break 15:00.
Also if you have not been running much for 3 years or so, then it might take you a couple of years just to get back to where you where.