Two speed sessions I used to do every week in summer in 2008 which got me to 16:15. 8x2 mins (3k pace) with 1 min recovery and 4x5 mins (about 15 sec slower than 5k pace) with 2 mins recovery. If you can record your distances (through a GPS watch) you'll see big improvements upto about 8-10 weeks, then the gains will plateau assuming you do not change other sessions.
Another similar session would be 6x1k with 1 minute. Drop the tempo run if you intend to train for 5k. Also try adding 5x100m strides to all non interval sessions.
Check out McMillian calculator for exact paces according to your best most recent time.