In the last several weeks before your goal race, your key workouts should closely simulate the demands that will be associated with the 10K-run leg of your triathlon. The most race-specific key running workouts are tempo runs at or near race pace, moderately long runs at a moderately brisk pace and transition runs (i.e. runs immediately following a bike ride), also done at or near race pace.
There are two reasons why these types of key running workouts should be emphasized late in the training cycle. First, the body adapts very specifically to the precise nature of the exercise demands placed upon it. Running at race pace prepares your body for race-pace running better than any alternative.
Doing overdistance runs at a brisk pace trains your body to finish strong better than longer, slower runs. And running in a pre-fatigued state after cycling in training prepares you to do the same in races better than any alternative.
The second reason you need to wait until toward the end of the training process to emphasize race-specific key running workouts (instead of emphasizing them throughout the entire training cycle) is that adaptations to any particular type of workout plateau after a while.
Therefore, it's best to develop a high level of general running fitness (as you did through the late winter) through less race-specific types of training before you begin to emphasize race-specific key running workouts. This will enable you to do these workouts at a higher performance level and, consequently, peak at a higher level of race-specific running fitness than you would if you started doing such workouts at a lower fitness level in the base phase.
The Build and Peak Phases
The portion of the training cycle that precedes the peak phase should be subdivided into phases that focus on other types of key workouts. The job of each phase is to prepare you for the next. In a hypothetical ramp-up for a July Olympic-distance triathlon, January marked the pre-base phase, February and March constituted the base phase, April and May make up the build phase, and June and July will be the peak phase.
Your toughest running workout in the build phase should be a weekly interval session. I usually advise triathletes to begin with speed intervals: a set of 30- to 75-second runs at roughly one-mile race pace with three-minute active recoveries between work intervals. Start with a very manageable number of repeats and build gradually from there.
After a three weeks of the speed intervals, skip the demanding session for one week, then move into three weeks of lactate intervals. The classic format for lactate intervals is 3-5 x (3-5 minutes at approximately 5K-race pace). Include a three-minute active recovery after each work interval. Again, start toward the lower end of the recommended times and distances and expand a little in each subsequent week.
At the end of May, the completion of the two-month build phase according to our hypothetical July goal event, phase out intervals altogether and start your peak phase with a weekly tempo run of as short as 10 minutes (not including warm-up and cool-down) and add two minutes to it every week until your peak-race taper (but be sure to take a recovery week every third or fourth week). Your longest tempo workout should not exceed 40 minutes or 10 kilometers, whichever is shorter. Throughout the peak phase of June and July, your tempo runs should be your toughest runs each week.
Long runs are also key workouts during both the build and peak phases. Throughout the two-month build phase, add a few minutes to your long run each week (except in recovery weeks, when, again, you should cut back) until you reach a maximum duration of around 90 minutes. In the June-July peak phase, include long runs of the same or slightly lesser duration and work on increasing the pace of these runs instead of their duration. This will make your long runs a little more race-specific in terms of intensity.
Brick workouts (i.e. bike-run workouts) and transition workouts are a topic unto themselves, and there's more than one effective way to integrate them into your training program. All I will say here is that you should regularly run off the bike throughout your training program and that in the peak phase some of this off-the-bike running should be performed at your anticipated race pace. Even a 10-minute race-pace transition run done once a week will be beneficial.
Finally, cut way back on all training in the final week to 10 days before your peak race and then show the world what a well-designed running ramp-up will do.