The light you see in the sky from a flash of lightning travels at 186,000 miles per second (~670 million miles per hour). But lightning itself travels nowhere near as fast as the speed of light. Although it moves at different speeds during different stages of its journey, lightning moves at approximately 3,700 miles per second (~13.3 million miles per hour).
Thunder travels at the speed of sound, which is relatively much slower at ~760 miles per hour. So you can quickly figure how far you are from the lightning flash by timing the pause between seeing the flash and hearing the thunder. When you see the flash, begin counting the seconds. Every five seconds equals ~1 mile in distance. So if you count 10 seconds between seeing the flash and hearing the thunder, you are about 2 miles away from the strike.