Try this workout for a mix up:

You can do this for swim, bike or run. But I am writing it up as a run workout.

Warm-up for 10-15 minutes of easy running. Then set your clock to 1 minute intervals. You will run at your 10k race pace for 1 minute and then settle back to your long run race pace for 2 minutes. Do this for 30-50 minutes depending on your level. This will add intensity and variety to an aerobic workout. Focus on a quick foot strike off the ground, relaxed face and shoulders and controlled breathing.