London marathon

My friends from Japan stayed at my flat for a week.

I checked it on the web and found that it would be held by chance on the Sunday when lthey staywedre in London by chancere.

So we chmangede our plans on Sunday tto see it.

No need to repeat "Sunday."

One of my friends had not seen Big bBen, yet.

Then,So we decided to go to Westminster Bridge, whichere wase locateould just beside London's famous clock thower Big bean,d to see the marathon.

We got stoff ate he tube at Leister Ssquare station and walked to Westminster Bridge.

By cWhancen as we passed in front of St Martin's in the Fields, we heard hymns comingby from inside the churanche.

ItThe was a tervimce towas starting and service...

Combine with the next sentence.

After enjoying the sounds, we went to Westminster Bridge through Trafalgar square and Admiralty Arch.

Capitalize names such as Trafalgar Square. (problem with program).

We took photos in front of them and Big ben, we moved beside of the course of the marathon.

"Them" is not clear. Better to say "
"We took photos in front of these landmarks as we waited for the start of the race."

It was not busy yet, because it we hasd so early to sriveed the marathon thour before; ithe was 9 pa. m. when is the starting time of the marathon.

When the first runner came, there wasere a dense crowd of people behind us.

WeFrom gotur a best position towe swere it and wble ctould see the runners whoin wthere fdistarnce from uas, they apprun toached. just iThen fronthey of us and turned to left before tighet in frosent of us.

The game was impressive.

This would be best at the end.

There were mMany runners run and many enthusimasticny crpeowdples cheeried along them conurse.

We were lucky to havbe an oppoartunity tof enjoy such fa grand estivalent.

London Marathon

My friends from Japan stayed at my flat for a week.

Although they were interested in the London Marathon, they did not know when it would be held.

"Proper nouns" are always capitalized in English.

I checked it on the web and found that, by chance, it would be held on the Sunday when they stayed in London.

So we changed our plans on Sunday so that we could see it.

One of my friends had not yet seen Big Ben.

Then, we decided to go to Westminster Bridge, which is located just beside Big Ben, to see the marathon.

We got up early to enjoy sightseeing before enjoying the marathon.

We got off the Tube at Leicester Square station and walked to Westminster Bridge.

Proper nouns again.

The hymns were played on a pipe organ and were so sublime.

After enjoying the music, we went to Westminster Bridge through Trafalgar Square and Admiralty Arch.

We took photos in front of them and Big Ben, then we moved alongside the course of the marathon.

"beside" isn't grammatically wrong here but "alongside" sounds more natural to an English speaker.

It was not busy yet, because it was only 9 a.m. which was the starting time of the marathon.

We got a good position at the corner of the course just behind TV crews and professional photographers.

When the first runner came, there was a dense crowd of people behind us.

We got a great position to see it. We could see the runners from far away. They ran just in front of us and turned to the left right before our noses.

The race was impressive.

"Game" is not really used for speaking of a race. Other choices could be: "event" or "competition"

Many runners ran and many, many people cheered along the course.

You could also say, "Many runners took part" or "Many runners competed".

We were lucky to have an opportunity to enjoy such a festival.

London Marathon

for titles of events etc we capitalize.

My friends from Japan stayed at my flat for a week.

Although they were interested in the London Marathon, they did not know when it would be held.

I checked it on the web and found that it would be held on the Sunday when they would be staying in London by chance.

So we changed our plans on Sunday so we could see it.

One of my friends had not yet seen Big Ben, yet.

So, we decided to go Westminster Bridge, which is located just beside of the Big Ben, to see the marathon.

We got up early to enjoy sightseeing before enjoying the marathon.

We got off the tube at Leister Square station and walked to Westminster Bridge.

By chance, we passed in front of St Martin's in the Fields and we heard hymns being sung by chance.

The hymns accompanied by the sounds of a pipe organ were so sublime.

After enjoying the music, we went to Westminster Bridge through Trafalgar square and Admiralty Arch.

We took photos in front of them and Big Ben, then we went and stood by the course of the marathon

It was not busy yet, because it was so early to see the marathon there; it was 9 p.m, just as the stating time of the marathon was about to start.

When the first runner came, there was a dense crowd of people behind us.

We got a good position to see it and we could see the runners who were far from us. They ran to just in front of us and turned to the left right in front of us

The race was impressive.

Many runners ran and many many peoples cheered along the course.

people is already plural so you don't need to add an -s.

We were lucky to have the opportunity to enjoy such an event.