Bricks
As a triathlete, running off the bike is a skill that must be perfected in training. Even though both sports use your legs, your body is in very different positions for each and the muscles that get used, as well as the range of motion for the two sports, are very different. There is a transition period that happens when you get off the bike and start running where your cycling muscles gradually stop trying to do their job and your running muscles start to take over. A brick workout trains you to make this transition quickly and efficiently.
When it comes to brick workouts, the main decision you have to make is the length of run you should do. An efficient brick workout should be a bike ride that is followed fairly quickly with a run of about 20-50 minutes. This is not an endurance-building run, but rather a neurological transition workout that is teaching your brain how to disengage your cycling muscles and engage your running muscles. The endurance you need to complete the actual run in your triathlon, even an Ironman, is gained from a combination of your over-distance training on the bike and from your long run workouts. Running more than about 8-10 miles off the bike can cause muscle breakdown that enters the unsafe zone. It is best to save those extended runs off the bike for race day when you are not going to be demanding that your body to go out the next day and train again.
If you have never done a brick, start by doing the run after a short ride, and then gradually transition to doing them after your longest ride of the week. It is not necessary to do a brick every week, especially if it takes more than a couple days of recovery to get your legs back to normal. When doing a brick, try to do the run before you let your energy level come down from the ride. You don't necessarily have to run the second you get off the bike, but don't wait until you're in front of the fridge looking for a snack while making a few phone calls. As long as you're running within about 15 minutes of finishing your ride, you'll be able to reap all the physiological benefits of the brick.
Speed Work
To run fast in a race you first have to run fast in your workouts. However, just like a brick workout, more is not always better when it comes to doing speed work. A total of 15-20 minutes of fast running in a speed session does the trick. Thirty to 40 minutes of speed work is too long to be able to go fast enough to garner the big gains that anaerobic training can give you. One of the markers that becomes elevated from doing speed work is your VO2 max, which is a measure of your ability to take up oxygen. The more oxygen you can get in for any unit of time, the more likely it is that you will be able to go faster. In running, the biggest gains in VO2 max come when you approach your maximum heart rate. (Cycling and swimming get those gains at lower heart rates.)
So let's say you are doing a speed session on the track where you are trying to run 10 x 1000 meters. You will likely find that because of the length of this workout you are only able to go fast enough to get your heart rate up to something like 160 beats per minute (bpm) rather than closer to your max heart rate (let's say 180 bpm for this example). Certainly you will be very tired from the workout, but you will not have gotten as much benefit out of it as you could have. However, if you shorten that workout to 5 x 1000 meters, now you will likely find that you can indeed push fast enough to get your heart rate up to over 170 on the last few 1,000's and you will get the biggest bang for your buck from the interval session.