When you run, lactic acid gets dumped into your blood from the muscles you are using. Your ability to clear the lactic acid buildup (lactate threshold) allows you to go longer and harder. When you are overwhelmed with the amount of lactic acid in your blood, you're forced to stop. Therefore, raise you lactate threshold and you will be able to run harder, longer.
Slow long runs helps you increase you max VO2, which is the amount of oxygen your body can consume during a maximal effort. Increase your max VO2 and you will be able to run harder, longer.
This is what I'm striving for in the spring/summer and should help me with my races (especially short ones). Below is an article I from Runnersworld.com on long runs.
Make Long Runs Work
by Owen Anderson
Researchers have been talking a lot lately about short, frequent exercise sessions. They say, for instance, that two 15-minute workouts will produce the same amount of weight loss as 30 minutes of continuous exercise and that numerous shorter workouts lower cholesterol just as much as fewer, longer sessions.
This research might make you think that you have no use for long runs unless you're training for a marathon. Or it might lead you to assume that you can split your long runs in half-say, running 8 miles in the morning and 6 miles at night-and get the same training effect as doing a continuous 14-miler.
Neither assumption is true.
Long runs have great potential to improve your performance, whether you run 5-Ks, 10-Ks, half-marathons or mara-thons. When you start dissecting the research, filtering out the health benefits and focusing instead on performance effects, you find that a workout lasting 35 minutes or more is definitely better than two or more shorter sessions that add up to the same amount of time.
During a longer workout you recruit more muscle fibers, fire up fat metabolism and even experience significantly higher heart rates than you would during a shorter effort. Long runs also fatten up your weekly mileage, which is good because higher mileage boosts your maximal aerobic capacity (max VO2) and strengthens your leg muscles.
Long runs also boost your endurance-your ability to run for long periods without stopping. To put it simply, a 10-miler on the weekend will give your fitness a stronger jolt than two 5-milers.
To make long runs work for you, however, you must do two things: Concentrate on intensity. It's your most potent producer of fitness. For a 10-mile run to really boost your fitness, you must do it at nearly your 5-mile training pace. If you run the 10-miler at a considerably slower pace, you won't get the maximum benefits.
Run at your goal race pace during part of your long run. The stamina gained from a long run is only relevant to the pace used during that run. If you complete 20 miles at an 8-minute-per-mile pace, you shouldn't have any trouble finishing a marathon at that speed. But you probably won't do as well if your marathon goal pace is 7-minute miles. In fact, a 10-mile run at 7-minute pace would help you reach your goal more than doing a 20-miler at a slower speed.
Don't worry excessively about the length of your long runs. Instead, focus on adding quality to them. You should cover "easy" miles within your long run at 90 seconds per mile slower than your goal race pace, then add quality miles at goal pace.
Here are some long-run suggestions to prepare you for different race distances. Make sure to start all of them with a good warmup. For the 5-K: Do 5 or 6 miles at a moderate effort, then a mile at 5-K pace, then 2 to 3 easy miles. This workout is like a fitness-spiking keg of dynamite, because you get the special benefits of long runs while enhancing your ability to run at 5-K speed.
For the 10-K: Run 8 or 9 moderate miles, then 2 miles at 10-K effort, then 2 miles at an easy pace.
For the half-marathon: Do 6 easy miles, then 6 more miles at your goal half-marathon pace, then an easy 2-mile cooldown.
For the marathon: Run 12 to 13 easy miles, then 6 miles at goal pace followed by a 2- to 3-mile cooldown.
Enjoy these runs. Covering 10 or more miles without stopping is a truly exhilarating experience, especially when you realize that your effort is having a great effect on your performance.
