I'm hoping you meant 30 minutes, not 300. You probably just pushed yourself too hard or become dehydrated. You need to pace yourself more. That's a long jog for your first time. Start out with a warm-up of a power walk for 10 minutes, then run for a bit, then slow down. Make sure to drink plenty of water and replace the electrolytes you lose through sweating--sports drinks (ones with low-sugar if you can find them) are good for this.

If you feel any pain in your chest or have a lot of trouble catching your breath, you should see a doctor. It could be exercise-induced asthma or something like that. It doesn't mean you have to stop running, but you should have an inhaler on-hand in case it happens again. If it isn't asthma, it could be something else that needs to be checked. Write down all the symptoms you experienced and any others that you have at other times. Indicate the severity and how long they lasted. Did you get out of breath? Was there any pain?

Just go a bit slower and I think you'll be fine. If it doesn't go away the next time or you have other symptoms, even when you slow down, you should see a doctor before you continue to work out.

Hey Venky,
This is a typical beginner problem so dont panic. I have been running regualarly for over 6 months now and I have passed many such hurdles.

Start slow. i.e Pace yourself as MyrahU suggested above.
If you have never jogged before or are ogging after a long time, you need to go slow. The best thing to do is walk-run which is very commonly used when starting out.

Walk-run means you start with a slow walk, then break into a run ( not too fast ...your speed should be where you can hold a conversation while running/jogging ).... then once you feel like you are out of breath , slow down and walk again... and repeat. You will easily be able to do about 2.5 miles ( 4 kms ) in 30 mins if you do this for a week.

Once you get used to that, then slowly reduce the amount of time you spend walking ( i.e recovery time ) and you wil see your pace get better.

One other important thing which most beginners neglect... Stretching is VERY IMPORTANT when you begin a jogging regimen. You must strech before and after your runs. You can find good stretching exercises online. It takes me all of 3-4 minutes to stretch but I no longer have shin splints or any other problems.

Good luck! Slow and steady wins the race. Enjoy jogging..dont punish yourself...

- Harsh