The issue: You fall apart in the final miles of the run.
The fix: For beginners this is sometimes an endurance issue, but for more experienced triathletes it’s frequently nutrition-related. You need to train your gut to handle more calories and adjust your nutrition strategy to support your expenditure. The exact amounts and composition of your nutrition strategy will be individual, but aim to get your carbohydrate intake to 25–35 percent of your caloric expenditure (if you’re using a power meter on the bike, your kilojoules value is approximately equal to your caloric expenditure). If you’re doing 700kJ of work per hour on the bike, you should be consuming 175–245 calories of carbohydrate, or 44–61g, per hour. Also, aim for 20–40 ounces of fluid and 500–750mg of sodium per hour if you’re not already meeting or exceeding those amounts.