There could be several factors and the first one that comes to mind would be that your body has not yet adapted to running these higher mileage runs. This can take some time so make sure your nutrition is on par (getting in some carbs--fruits and veggies are carbs) as well as making sure you are getting plenty of sleep.
For most people, they have enough glycogen to fuel them through a good 2 hour run without the need to refuel--remember your fueling that you are consuming during your run is not restoring your glycogen stores but keeping your blood sugar stabilized without dipping into your glycogen stores.
The problem is with refueling too soon or too frequently during your training runs (races are a different scenario and I'll touch on that in a bit), is that if you do not drain your glycogen stores your body does not have the need to make the tanks bigger (think of it like you topping off your gas tank every time you pass a gas station). This is why you are training--this is why you need to go through some uncomfortable runs (Last week's runs for me STUNK big time--and today I ran my third fastest half).
So what are some possibilities for your issues--as I mentioned earlier it could be that you body has not adapted to your new distances so you are going to feel some fatigue. It could also be that you are running the first couple of miles too fast--you want to slowly and progressively build your pace--even if the first several miles are way below your training pace (during my LSD runs my pace is easily 2 minutes SLOWER than my finishing time)--the reason you want to run slower in the beginning is to conserve the glycogen in your muscles and liver BUT also to teach your body to burn fat for much of your fuel. Once again this is why we train.
The more you run, remember you are also building a nice network of capillaries which will supply your muscles with the nutrients and oxygen your muscles will need to keep you going...Once again this is why we train.
Lastly, if you are not refueling IMMEDIATELY after your run with some carbs and protein--I have fruit and Greek Yogurt after my run--you are missing out on a prime opportunity to replenish your glycogen stores (that you just trained to empty) because your insulin sensitivity is at an all-time high the first 15-30 minutes after your run and it is eager to shuttle in some carbs into those glycogen storage tanks. And the protein you just ate will begin to help the repair process to those damaged muscles.
So what about racing and why do runners refuel? When you get to your longer training runs (more than 2 hours) than you may then want to start experimenting with refueling sources--while some people like the gus and stingers and shot blocks, the reality is, that any form of sugar (StarBurst, skittles, even pretzels) can do the trick--the only thing that gives the gus, blocks and stingers an advantage is the convenience factor--but remember you MUST have access to water when you take them to help usher them out of the gut.
When you race, the goal is to run at a slightly faster pace than you did when you were training--you should not be training all your long runs at race pace (that is why progressive runs work well--it teaches your body to feel the speed of race pace without running the whole distance at race pace which can leave you with a potential injury or overtraining issues). The faster we run, the more dependent we are on glycogen to support our race pace--in other words our body is going to start using some of the stored glycogen. When our body's blood sugar level begins to fall, it will start turning to the glycogen stores (that's what you were training to do--making those storage tanks bigger) to stabilize your blood sugar level. The trick is refuel early on--about 45 minutes into your run and then every 15 minutes thereafter and also take water with it.
Now I do want to caution you that it is NOT, I repeat NOT unusual for you to gain a little weight as your mileage goes up--the reason--for each gram of stored carbohydrate your muscle and liver has, your body will ALSO store about 3 grams of water along with it. The reason, your body hangs on to more water to help with the cooling off process for exercise and for helping process energy. Your muscles will also have an increase in blood volume in order to have better availability to oxygen and removing waste, especially lactic acid.
Now, the longer (year-wise and month wise) you have been running, the less dependent your body will be on refueling during your training runs and races--the reason, you have taught your body to handle the blood sugar stabilization through your consistent training. I have not refueled for the last 10 half-marathons (I do drink water though and will take some Gatorade from time to time--mainly for the electrolytes) and have done just fine.
I hope this helps!
Edited by: SP_COACH_NANCY at: 4/6/2013 (15:45)