To train effectively for endurance events, you have to build your endurance, and the way to do that is with long workouts. When training for marathons and triathlons in particular, long runs will give you the endurance to go the distance on race day.
Want a recipe for marathon success? Be sure to include a long run once a week for 10-12 weeks. The length of that long run varies, but most marathon training plans advise 2-3 training runs of at least 20 miles during the few months before the marathon, depending on whether your goal is to finish or to finish faster.
The key is to build up gradually and allow your body to adapt. Assuming that you are running at least 20 miles a week for 5-8 miles at a time, start by running 8-10 miles one weekend. Then follow this guide:
Week 2: Run 10-12 miles
Week 3: Try for 12-15 miles.
Week 4: Drop back to 10 miles.
Week 5: Try for 15-17 miles
Week 6: Go for 20 miles, but keep the pace easy.
The week after a 20-mile run, make your "long run" 10-13 miles, or less if you are new to long distances and you still feel tired. Repeat this pattern at least one more time before the marathon, working up to a second 20-mile run and then backing down. Some trainers advocate working up to 26 miles in training prior to the marathon, while others contend that your long training runs need not exceed 20 miles. If you want to work up to 26 you can, but don't do a run that long closer than 3 weeks to your "official" marathon.