You’ve gotten past the swim and cycle and gotten to the the run and you’re doing the triathlon waddle and trying to accelerate like a hamstrung moose on downers. Most triathletes do not go into a controlled run and instead labor until they get their legs under them. This may significantly hamper the last stage of your race.
A controlled run is one in which you establish a pace for each mile or time mark. It is establishing the practice in your physiology and psychology that pace is determined by a plan and not by emotion or sense of well being (although those play roles as well.) This is a concept difficult to be grasped by novice and veteran runners but is also something that would work extremely well with them as well. An example of a controlled run is one in which an athlete will run 9 minute miles for the first two miles of an eight mile run then slowly increase to 8's on the third mile and maintain for the next four miles then run a 9 run the last mile. Essentially this is a warm-up, a tempo run, and then a cool down. Another example of a controlled run is one in which a coach will determine your pace for you at different points.
Elite level athletes will establish a competitive but controlled pace for the first mile of the run. Hunter Kemper was fantastic at this and he would plan out a specific pace post bike that would allow for his aerobic and mechanics to adjust to the run but even he found it difficult to make sure he didn't wind up in the "chase". The concern of a chase is that given your energy expenditure in the swim and bike, the most drain occurs during the run (and at the highest rate) - essentially if you not allow for adaptation you're draining your batteries quicker. Only repeated practice using controlled runs instill the disciplined needed.
Practice controlling each run. Have a plan for every run you do (even the casual ones.) Make it a habit. When you reach the triathlon it will not be a matter of heart or good weather or fear or leaving much of yourself on the bike - it will be a matter of your plan and executing it and allowing for your body to adjust. And it will.