I used to do this workout.
2000, 1600, 1200, 800, 400, 200 (rest getting 30 seconds shorter after each interval: 3:00, 2:30, 2:00, 1:30, 1:00)
I would run the 2000 at 1 second slower than what I thought 5k race pace could be at that current time. With each interval I would get .5 - 1 second faster per 400 for each interval, till the 400 and 200.
Example 5:40(68), 4:30(67.5), 3:21(67), 2:12(66), 64, 28
I always felt that the pace between the 1600 and 1200 was a great predictor of what I could run for a 5k.
The 2000 was tough but just made you tired. The 1600 was certainly tougher. The 1200 was always the toughest part of the workout though, and really simulated the feeling of a 5K with 1200 to go, as did the 800.