This can be a few things...

The first and simplest things you can check is your shoes. Next time you go out and run loosen them up a bit. If they're too tight they may be cutting off the circulation.

The second thing is you could just flat out be out of shape and it's hitting you hard.

Third thing which I have, Is called compartment syndrome, common in runners. Basically what happens is when you start running or working out blood rushes to the muscles in the leg and causes the muscles to natural swell. There's tissue in between the muscle and the skin called the fascia. Basically if the fascia is too thick or because of an imbalance in muscle growth and blood flow what happens is the nerves or blood vessels in your leg can get pinched off causing it to go numb or cause pain.

For me it occurred when I started training again just after off season from soccer. Cardio workouts that build up your stamina can keep your heart rate down and increase your stamina before it starts going numb. As well as stretching. I rarely get it anymore unless I'm doing really hard running. The other two options are therapy and surgery. Therapy is pretty self explanatory. Surgery involves cutting open your leg and removing some of the fascia. Pretty simple procedure. If you're worried about it go talk to a doctor and they can do a simple test, although it's not the most comfortable. A doctor will also tell you if this can be a serious problem.

I'd recommend checking your shoes first. If that still doesn't do it keep working out and see if as you do get in better shape it comes on as quickly. If it's still limiting your abilities go see a doctor because it can cause damage if it's really bad and continuously happening.