What I say here depends on what your run raining and performance are like now.
The bad news is that many triathletes have a very weak true running base fitness. The good news is the solution, depending on back ground is very easy - run a bit more, but run a bit more in the RIGHT way.
For most that means doing a block of training where you run almost every day of the week, utilizing what around here is commonly referred to as the BarryP formula, although, I have been a big advocate for this as well for a long time.
If you do have that classic weak running base. If you follow the higher frequency formula of running 5 - 7 days/week for a couple of months, the right way, and then taper, for a week, almost everyone that does this, records a 5k or 10K PB at the end of it. They'e done no fancy training. No higher level interval workouts - they have just got out and got in a run of at least 20 minutes almost every day of the week. Some runs a are a little faster, some a little slower. Just run based on feel. Run hard enough that you will be able to run the next day, and the next day and the . . . . you get the picture.
Steve Fleck @stevefleck | Blog