Corrections/Mistakes/Suggestions

Those quiet ten minutes in(/of) fear

I'd want(/would like) to tell a fact(/story) that happened some years ago, while in one August, I was in Corsica for a short holiday with my wife and a couple of friends. One morning we decided to go for a trekking tour along the Restonica's Gorge, a very nice canyon, near the town of Corte, in the center of Corsica. It is a walk of about one hour and a half (that you can finish when you arrive(/that it will take you)) to reach Melò's lake at 1700 metres of altitude or you can continue until you reach another lake, that is situated 200 metres more up, at 1900 metres, (which is) called Capitello's lake.

It was a lovely day: we (halted) at(/stoped by) Melò's lake and there we made a swim. After (the) lunch, we decided to go back to the Hotel in Corte. While we were going down along the(/a) rocky wallhillside, the weather began to change: all of a sudden it seemed like the weather was(/had been) divided into two parts: while we were (still) in the sun, we saw an hail storm coming towards us and (in a short time)(/(pretty) soon) we found ourselves (right) within (of) it, passing sowith temperatures dropping from about 30°C to less than 20°C in a time of a few seconds(/within a few secounds).
Paraphrasing(/Quoting) an old movie, I can(/should) say that we quiet had our ten minutes of fear, because the storm was very strong and violent.(..fear, since how strong and violen the storm had been)
As quickly as it had arrived(/appeared), so quickly(/swiftly) it went away(/went by/disappeared), and the sun returned, to shine again.

I advise to(/My advice) visit Corsica because it is a beatiful island, but be carefull with the weather : it may be unpredictable(/it is damnit upredictabele...;P)

Those quiet ten minutes in fear

I'd want like to tell a fact happened some years ago, while in one August, I was in Corsica for a short holiday with my wife and a couple of friends. One morning we decided to go for a trekking hike along the Restonica's Gorge, a very nice canyon near the town of Corte in the center of Corsica. It is a walk of about one hour and half that you can finish when you arrive to at Melò's lake at 1700 metres of altitude or you can continue until to you reach another lake, situated 200 metres higher more up, at 1900 metres, called Capitello's lake.

It was a lovely day: we halted to Melò's lake and there we made a swam. After the lunch, we decided to go back to the Hotel in Corte. While we were going down along the rocky wall, the weather began to change: all of a sudden it seemed that the weather was divided in two parts: while we were at sun the  (in sun ?), we saw an hail storm coming towards us and in short time we found us within of it, passing so from about 30°C to less than 20°C in a time of a few seconds. 

Paraphrasing an old movie, I can say that we had our quiet ten minutes in fear, because the storm was very strong and violent.
So as it was arrived, so it went away, and the sun returned to shine again.

I advice to visit Corsica because it is a beatiful island, but be carefull at  with the weather : it may be unpredictable

Those Quiet Ten Minutes in Fear

I'd like to tell a true story that happened to me some years ago. It was August when I was in Corsica for a short holiday with my wife and a couple of friends. One morning we decided to go for a hike along the Restonica's Gorge; a very nice canyon near the town of Corte in the center of Corsica. It is a walk of about an hour and a half that stops at Melò's Lake, at 1,700 meters, or you may continue to another lake, situated higher at 1,900 meters, called Capitello's Lake.

It was a lovely day, and we paused at Melò's Lake for a swim. After lunch we decided to go back to the Hotel in Corte. While we were going down along the rocky wall, the weather began to change. All of a sudden the weather was divided in two. Above us the skies were clear, but we could see a hail storm coming towards us. Before we knew it, the storm was upon us! The temperature dropped from  30° to less than 20° in just a few seconds.
Paraphrasing an old movie, I can say that we had our "quiet ten minutes in fear", because the storm was so very strong and violent, but as soon as it arrived, it was already gone, and the sun was shining again.

I can recommend a visit to Corsica because it is a beatiful island, but be carefull of the weather; it may be unpredictable!

McAustria's edit is correct. I edited your narrative into "American English" for you. 

Good story! Good studying!