Tired legs are no surprise after any running session, even if you are only moving at a light jog. Even if your legs get tired in the middle of your run, it's just your body responding to the exercise and is perfectly normal. The main culprit is a buildup of lactic acid, but underdeveloped leg muscles and a too-intense pace can also contribute to the gradual heaviness that can make it difficult to go any further. In most cases, pace control is all it takes to lighten up.
What's Normal
When you run at a pace above your anaerobic threshold, which is the point where your muscles use carbohydrates instead of fat for fuel, lactic acid is produced as a byproduct of the energy conversion process. As the lactic acid accumulates in your muscles, they begin to feel heavy and cumbersome -- but they recover quickly after a few minutes at a slower pace. This is why a short sprint can make your legs more tired than a long jog -- because the sprint causes lactic acid buildup and the jog uses fat for fuel.
Training Too Hard
It doesn't necessarily take a sprint to bring on the lactic acid. Anaerobic threshold varies per person, and can be as low as 55 percent of VO2 max in someone that is relatively new to exercise. This means that it is possible for an otherwise comfortable jog to stress your muscles to the point where they switch to carbohydrate fuel and produce the lactic acid. If you keep up with the jogging, you'll notice that your anaerobic threshold will gradually rise, and you'll eventually be able to run faster for longer periods of time. This is the whole point of endurance training.
Running Too Fast/Hard
You may also just be running faster than you think. Many runners don't bother to log their miles or track their pace, and if you've gotten into the "groove" of running, you may be working harder than you realize. A jog is generally considered to be about 5 mph, which is only slightly faster than a brisk walk. If you're running faster than this, you may be stressing both your muscles and your cardiovascular system more than a jog would, and it would be perfectly normal for your legs to get tired. The same holds true if you have recently switched to trail or beach running, because the uneven terrain works slightly different muscles than road running.
Being New
If you are completely new to running, it is normal for your legs to get tired, even at a jogging pace. Not only is your anaerobic threshold probably quite low, but the "running muscles" like your calves, hamstrings and quads are not used to the stress. Adding a twice-weekly strength training routine for your legs will help you build up strength for better propulsion, and your muscles won't have to work so hard to move you forward. Losing extra weight helps too, because the less excess fat you have, the lighter the load on the leg muscles.
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