I was just reading an article on Active.com on how the top age group duathletes can run 
and ride so fast. Consider this:
At the 2010 USA Triathlon National Duathlon Championships in Richmond, Virginia, the age-group champion and overall fastest time was 1:30:39 for a course that consisted of a 5K run, 38K bike and 5K run. Kaleb VanOrt, a 26-year-old from Mishawaka, Indiana, maintained a 4:55 pace on the first run, a 24.05 mph pace on the bike and finished with a 4:59 pace on the second run. His margin of victory was 26 seconds over the course of the race.
Kaleb had the fastest run split of the day, by over 42 seconds, while his bike time wasn’t even in the top five, what does this tell you? You have to be able to run fast off the bike on tired legs. And then consider this:
Another key to the incredible splits we see from the top duathletes in each age category is a deep aerobic fitness base. The offseason base miles, especially on the run, done at an easy-to-moderate pace help promote aerobic efficiency and build strength in the skeleton and connective tissues when combined with nutrition and recovery. Speed work doesn't work without the base miles.
No base to build on, forget about building sustained speed. Endurance, power, and speed, these are the three things we are building when we train. Endurance, can I complete the distance, regardless of pace? Power, can I complete the distance over a hilly course, or against a strong headwind? Speed, how much faster can I complete the event?
Off and early season base miles, done in HR Zone 2, “conversation” pace, ignored and misunderstood by many, especially those looking to race fast ASAP, give us a base from which to build the power and resultant speed we’re searching for. The lesson here is if you don’t have a big enough endurance base and you think you can just cram on some speed training just before your next race, you’re probably better off going longer and slower, while incorporating some tempo work, instead of actual speed work. And one last comment:
A final key to Duathlon speed is learning to run well on tired legs. The gritty reality of going fast is you simply have to train hard even when you are tired. There is a difference between being over-trained and training when tired and physiological markers such as resting heart rate can provide that perspective. The faster athletes are the ones with the deep fitness base and a lot of speed work in their training log. Like all endurance sports, there are no short cuts to high speed.
Just what you wanted to hear huh? You’ve trained long and slow, built power, done your speed work if/when time allowed, and now we have to go back out and train hard on tired legs, when you’re brain is thinking rest/recovery day. The key here is to note the reference to resting heart rate, one sign of overtraining, and to make sure before you add this last layer of training, that your body is able to handle the increased stress.
Nobody said it was going to be easy, and most of us can only dream of running sub 5:00 pace at any time, no less during a couple of 5k runs that bookend a 38k bike in a Duathlon. The keys here for everyone, not just the really fast guys, are the simple steps outlined above: base work before speed, training hard on tired legs. Add these to your To Do list for next season and see if you don’t improve on your run splits.
Fred Maggiore, USAT Level I Coach