Legs tired before Im truly tired?

Question
I'm 18, female 5'2 around 115 lbs, healthy and currently taking my cardio more seriously (training for a 5k), along with weight lifting. So in the past 2 months I've been running mainly on the treadmill at relatively high intensity for about an hour 3-5 times a week. I've noticed really, really good results and improved performance/endurance until about around 2 weeks ago.

My endurance really has gotten better and I don't get winded nearly as quickly as I did before and my heartrate isn't so incredibly high. But now I've found that it's my legs that can't seem to "keep up." Halfway through they start to get pretty tired (mostly just my calves), but I'm positive I can still keep going because my body is not winded or completely tired. I have to slow down to a walk so they can rest and then start back up again and I think it's getting in the way of overall progress.

I've been doing more leg oriented exercises (my lifting revolves mostly around the lower half now) because I thought maybe my calve muscles can't handle it but I haven't really seen any (even short term) results. Now I'm starting to think it could be a running form issue and lack of sufficient muscle, but if it is I'm not sure how to go about fixing it. It's a weird imbalance I think. Are there any specific leg exercises I can do? How do I even know if my running form is wrong?

Answer
Arial:

At your height and weight, you are certainly not overweight.  So I'm going to assume that your running program is mainly about increasing your level of your physical fitness, and being able to finish the upcoming race with a respectable time.

Although I haven't actually seen your run, I don't think your stride is the problem here.  A good running stride usually taxes the calve muscles, and since this is where you are feeling it, we need to look elsewhere.

Here is what I think is going on.  You are exercising intensely to compete in your upcoming race.  (Perhaps too much - an hour on the treadmill seems like an awful lot for a 5k!)  But your hard core training is being hampered by your not eating eating enough of the right types of food at the right times.  When insufficient blood glucose is available to produce energy, your body must shift to other forms of fuel, such as breaking down adipose tissue (fat) and/or utilizing glycogen, a chemical stored in your muscles and liver.  

It is quite possible for an athlete to exhaust their entire body's store of glycogen, particularly when doing semi-anaerobic exercises such as interval training, sprints, or training for short, fast-paced races like a 5k.  When a muscle's glycogen reserves are depleted, it gets that heavy, dead feeling and can't seem to do any work anymore.  Basically, it's out of gas.

Fortunately, there is a very easy way to combat this, especially for a 5k race.  10-12 minutes before you start your training (or the race itself), consume a high-glycemic carbohydrate (HGC) beverage.  HGCs are things like glucose (also called dextrose), maltodextrin, and the dreaded fructose.  The reason why fructose has gotten a bad rap is because it is absorbed into your blood very rapidly, which forces your body to produce lots of insulin.  For a sedentary person, this is a bad thing.  But for someone about to go for a 5k run, it is PERFECT.  The extra sugar floating around your body is preferentially utilized before glycogen (assuming adequate respiration).  Likewise, when you finish your training (or race), you can also consume a HGC beverage, ideally with some easy-to-digest proteins such as whey or soy mixed in.  Doing this will help your muscles recover much faster from training.  This is one of the ONLY times I ever advise people to consume sugar (HGCs)!  This same trick can also be used when you are doing the strength training exercises for your legs.

Doctors John Ivy and Robert Portman discuss this sort of thing extensively in their book "Nutrient Timing".  In case you care not to read the book, the basic idea is that timing intake of proteins and HGCs can boost training results to a significant degree.  For someone in your weight range, the pre-race drink should contain about 22g of HGCs.  The post-race drink should consist of about 40g HGC, and 14g protein.  For a very long race (such as an hour on the treadmill), you can drink a little more for your pre-race drink, or else take sips of one while running.

Note that drinking a soda is NOT ideal, nor what I am saying to do.  Soda contains only fructose, and you are really better off with a drink containing a mixture HGCs.  Also, one can of soda contains around 40g of carbs - too much for your pre-race drink.  Check out a health food store and look for pre-workout nutrition supplements (they may be in the body building aisle.)  They typically come in powders that you can weigh out and dissolve in water yourself.

Good luck!