Riding a Bicycle in Seoul, Korea

I recently bought a bicycle and I try to ride on it whenever I get the free time. There is a very well made built bicycle road track along the beautiful Han River, so I go there to ride. I rode my bike for about one [please write out numbers alphabetically] and a half hours today. This really helps me to stay fit.

In the At elementary school I learned that 70% of Korea is mountainous ground( I mean not plain/flat). So most of the cities and roads are made by cutting through these mountains. Because of that, there are so many a lot of uphills and downhills throughout the city. Plus, Seoul is a really very crowded city so it's very dangerous to ride a bike in the city of seoul. All these facts explain why most riders in Seoul come to the Han River for a ride. It's the same logic for skateboarders or rollerbladers.

Imagine bicycle cyclists, skateboarders and rollerbladers all using the same 'bicycle road track'. It's very dangerous.

I feel pity sorry for all these leisure people that is growing rapidly anyone who wants a relaxing time, soon the Han River will be also crowded by many with riders. And the freshness peacefulness of riding along the river would will become substituted to a stressful. I hope that day comes slowly.

*your correction is so much appreciated!
Please correct not only grammatical errors, but also unnatural word usage. you may rephrase the sentence to much more natural way. Any advice would be so helpful to me.*

Riding a Bicycle in Seoul, Korea

I recently bought a bicycle and I try to ride on it whenever I have get the free time. (You could say 'have free time' or 'have the time', both are correct.) There is very well-made bicycle road trail along the beautiful Han River, so I go there to ride. I rode my bike for about one and a half hours today. (Either say one and a half hours, OR 1.5 hours. The use of numbers is less formal, but it's totally okay in this context, because you're not writing a formal essay!) This really helps me to stay fit.

In the elementary (A less formal way of saying 'elementary school' is just to say 'grade school'. It sounds more natural, but I haven't corrected it because it's not wrong!) school, I learned that 70% of Korea is mountainous ground( I mean not plain/flat). So most of the cities and roads are made by cutting into the these mountains. Because of that, there are so many uphills and downhills throughout the city. Plus, Seoul is very crowded city so it's very dangerous to ride a bike there. in the city of seoul (Because you already mentioned Seoul as the subject of the sentence at the beginning, you don't need to repeat those nouns!) All of this these facts explains why most riders in Seoul come to the Han River trail for a ride. It's the same logic for skateboarders or rollerbladers. (This sentence isn't necessarily incorrect, but I might say 'The same applies to skateboarders and rollerbladers.')

Imagine bicycles, skateboards and rollerblades using the same 'bicycle trail'. It's very dangerous.
I feel pity for all these leisure people that is growing rapidly. (I can't quite figure out what you're trying to say here. Maybe 'I pity all the people who try to enjoy the trail.'? I'm not sure it's necessary though, so I might just delete the sentence if I were you.) Soon Han River will be also crowded with many riders. And the freshness of riding along the river will be stressful. I hope that it doesn't happen too quickly!

*your correction is so much appreciated!
Please correct not only grammatical errors, but also unnatural word usage. you may rephrase the sentence to much more natural way. Any advice would be so helpful to me.*

(Hey! So you say bicycle a lot, and I wanted to point out that you can also shorten that word to 'bike' to make it sound more natural. Like 'bike trail' or 'ride my bike'. I haven't corrected that above, because it's not wrong, but that's just something you can keep in mind when you speak/write! ^__^)

(I point out a few things, in this blue above, that might help you sound more natural... little comments here and there. If you have questions, don't hesitate to ask!)