What are some marathons i can run round the world i want a large crowd
Sure, look below at the largest marathons in the world you can run in as a member of the public. 

1. New York City Marathon
New York is a major city in the United States as well as the world. Its marathon is no different. Each year, the New York Marathon brings in up to 50,000 runners. The starting places are strictly regulated, however, and there is a split quota between New York residents, Americans, and anyone else. This means your chances are slim if you are not from New York or America, but you can purchase tickets with a tour company, which may be the best way for non-USA citizens to attend. The marathon is run around early November, and anyone over 18 is welcome to apply.

2. Chicago
Staying in America, the next marathon on the list is Chicago, which is in the state of Illinois. Regularly drawing 40,000 participants, the Chicago marathon is a great way to push yourself to the limit and see one of the United States' most populous cities. It is a bit easier to get a ticket for the Chicago marathon with no qualifying times and an entry age of just 16. The registration closes at a max of 45,000. The Chicago marathon takes place in mid-October.

3. Paris
The city of love, Paris, is an incredible place full of sights, smells, and sounds. In early April, it is also the host of the third-largest marathon in the world. About 40,000 descend on France's capital to test themselves in the streets where Napoleon used to walk. Entry differs from the USA marathons, with participants needing a sports health certificate signed by a doctor. You can apply online or in writing; the minimum age is 18.

4. London
A historical city like no other, London hosts one of the most sought-after marathons in mid-April each year and boasts around 35,000 crossing the finish line each year. Any foreigners will struggle to get a starting place for this famous race, with 7% as the reported number for how many foreigners finish each year. The best and easiest way to get a ticket for the London marathon is to purchase one through a travel agent.

5. Berlin
Where better to run 26.2 miles than the German capital of Berlin? Take in the war monuments seemingly on every corner while you compete with a limited 40,000 people to get around Berlin the most quickly. As with the London marathon, the best way to get a ticket for the Berlin marathon is through a travel agent or charity provider.