A basic set of 4 X 800 can be adjusted as you progress through the build phase in your training. The 800's should be run at or just below your lactate threshold heart rate or a perceived exertion of 7/8 on a scale of 10. Remember that running a little above your LT for an extended period of time, even by only a few beats, can do a lot more muscular damage and require more time to recover from the workout. Get more bang for your buck with a little control in order to allow room for subsequent quality workouts. The rest interval should be half the distance of the work interval (400 meters) and jogged. The 400 meters can be shortened to 200-300 meters rest as you progress through your build phase. The number of 800's can also increase in proportion to your fitness. Being able to maintain your heart rate at LT through a higher volume indicates an increased level of fitness.
As triathletes, we mustn't forget to simulate running on tired legs. Instead of calling it a double brick workout, which sounds terribly daunting, I prefer to call this workout the quadruple chocolate fudge layer cake. You can decide which is the fudge and which is the cake. Set up your bike on a trainer at a track or other site with a measured, out-and-back or circular course. Warm-up on the bike for 15 minutes elevating your heart rate to zone 3. Transition to the track for a 400-meter run at 10k pace. Transition back to the bike for a 5-minute ride in which you spin easily for 2 minutes to recover and then build heart rate to the 4-5a zone and hold for 3 minutes. Transition to an 800-meter run at 5k race pace. Repeat this pattern of 5 minutes on the bike followed by an 800-meter run 2-3 more times. After the last one, cool down on the bike by spinning easily for 10 minutes. This workout will take approximately 1 hour and should be done during the build 1, or 2 phase (3/4 of the way through your training for a peak race).
Another great track recipe, done with or without the bike, is a set of 6 X 1 mile at 10k race pace or slightly faster. The recovery should be a 600 meter jog or half the work interval time. For example, a 6 minute mile pace recovery would be 3 minutes. If you want to 'layer' the workout to simulate race legs, add in a 40 minute ride prior to the run. Build up to zone 3-4 and maintain the effort in this zone for at least the last 15 minutes. Try to run each mile at about the same pace. Test yourself a couple of weeks later to see if you can sustain a faster pace for all 6 repeats. This is either a confidence booster or a reality check. Is that pace you were planning to run in the race realistic? Far fetched? Too conservative? Consistency is the key factor in this workout.
The track is one of your training appliances, consider it your blender. Play with the ingredients and speeds, but be smart about gradually increasing volume and decreasing recovery as your fitness progresses. Be your own Iron chef!