There were many different types of running races in the Greece olympics. Some of them are the stade,the diaulos, and lastly the long race. The rules- all the runners had to start at the same time. If a person ran earlier than everyone else then that person would get beaten or disqualified. A person knows when it's time to race when they say a specific word of command.
Back then Greek runners used to wear some clothes but then they decide to go fully naked while running.
running was the most practical and common form of exercise and sport in Ancient Greece
This is similar today in many ways. One way is because every racer has to start on time. Also all athletes trained or had a personal trainer. That is how running in the olympics back then is the same as today.