I'd definitely agree with @solomongaby on landing below your COG. To answer your question directly, a forefoot strike can and should be effective at any pace. Bear in mind that the biomechanics (and resulting efficiency) of your strike has everything to do with your carriage. To that effect here are a few things to keep in mind:
- Keep your posture aligned with ear, shoulder, hip and ankle in a straight line
- Strive to lean forward at the ankles and not the hips or waist. Think of falling like a tree
- Aim for a quick turnover and shortened stride with the heels pulling up the buttocks at roughly a 45 degree angle. Just as solomon said, this means lengthening your stride behind you.
- Try to run as light as possible (to see what I mean, attend a local race and head go to the finish line and listen to the finishers. The first runners to cross the line will be very light-footed...nearly silent. As more of the field finishes, the runners steps will be louder and heavier. I've always been coached to run as if I'm on sand and trying to leave the lightest print possible)
Also, another visualization that has helped me tremendously is to imagine you have dowel rods extending from the inside of each ankle. Naturally, you'll need to step over each rod in order to keep from tripping.
Since this is a lot to swallow I'd also suggest picking one aspect of your form to concentrate on at a time or per training session until you get it down. For a more in-depth look into techniques, exercises and the science behind proper form and subsequent forefoot striking, the Chi-Running (Video Overview of Chi-Running) and POSE methodologies (Comparison of Heel vs Forefoot strike and Form) of running are great to learn and study. Both have videos and written literature available. I don't recommend any one over the other as the both generally teach the same style of running but with different terminology.
Lastly, try 20 - 30 minutes of barefoot running on soft grass. When you remove the barriers of thick soled shoes, it's easier to "feel" proper form and foot strike.