I was worried for suffering hypothermia, so I wore two more wearlayers plus a raincoat.

I didn't know if it was actually foggy, or I was inif it was because of the misty rain.

Or I was just in the clouds because the altitude of that place was very high, but not that high as I noticed thin air.

The uphill's and downhill's were so steep, very wet, and so slippery that I had to walk instead of run.

Originally I started running for my health, but if I fell to diemy death or got severely injured, my attempt turned out in vain completelywould have turned to completely be out in vain.

I knew well from my recent actual experience that I tended to fall quite easily, so I knew I mustthat I should be very careful.

Yesterday, I ran (a) 10k in the mountain infor an official trail -run race in 1:42:36*, **with thean average heart rate of 151 bpm, with the and an average running pace of 9:42 per kilometer, spendburning 1,104 kilocalories*** in total.

*This is OK, but normally we will write the numbers out: one hour, forty-two minutes, and thirty-six seconds.
** This sentence is very long. I would personally end the first sentence here, and start a second sentence here.
*** I'm not sure why, but in the US, we only say "calories"

It was chilly out in the rain.

could also say "because of"

I was worried forabout suffering (from) hypothermia, so I wore two more wearextra layers plus a raincoat.

I thought I was going to be a "Cold Hot" Dog"... that was distastefula very dry joke.

However, I became a "hot" dog again while running, so I took off themy raincoat and a sport jacket.

In English, articles of clothing are normally used with possessive pronouns — my, his, her, etc

However, ironically, we were surrounded by thewe were in the midst of a deep fog, and I couldn't see any of the beautiful scenery there, and I hardly saw the runners ahead of me, with the potential fear of getting course-out andso I was worried about potentially going off course and getting lost in the mountain.

I didn't know if it was actually foggy, or if I was in the misty rain...

Or if I was just in the clouds because the altitude of that place was very high, — but not that high, as I noticed thin air.

— (em dash) can be used for many things, including interruptions and showing thoughts

The uphills and downhills were so steep, very wet, and slippery that I (often) had to walk instead of run.

Originally I started running for my health, but if I fewere to fall to dieath or goet severely injured, my attempt turned out in vainefforts would end up completely in vain.

You're suggesting a hypothetical situation here, so you need to use a conditional tense. It's the same as "if I were rich, I would ___"

I knew well from my recent actual experience that I tended to fall quite easily, so I knew I must be very careful.

Yesterday, I ran 10k in the mountains in an official trail run race in 1:42:36, with the average heart rate 151 bpm, with the average running pace 9:42 per kilometer, spending 1,104 kilocalories.

In the US, (not sure about the UK) kilocalories is just called calories.

I was worried forI'd suffering from hypothermia, so I wore two more wearlayers plus a raincoat.

In causal speech, people just say "I'm worried I'd get hypothermia"

I thought I was going to be "Cold Hot Dog," that was distastefulwhich would not taste good.

distasteful is basically like 趣味悪い

However, I became "hot" dog again while running, so I took off the raincoat and a sports jacket.

The scenery of the mountains would have been terrific beyond my imagination if there had been no fog.

However, ironically, we were surrounded by the deep fog, and I couldn't see any beautiful scenery there, and I hardly saw runners ahead of me, with the potential fear of getting off course-out and lost in the mountain.

I didn't know if it was actually foggy, or Iif it was injust the misty rain.

The grammar is right but this sentence is a bit more parallel so would sound smoother.

Or I was just inmaybe it was the clouds because the altitude of that place was very high, but not thatso high asthat I noticed the air thin airning.

The "or maybe" change is totally optionally, but it would transition a little better from the previous sentence.

Originally I started running for my health, but if I fell to diemy death or got severely injured, my attempt would turned out in vain completely.

Yesterday, I ran 10k in the mountains in an official trail run race in 1:42:36, with thean average heart rate of 151 bpm, with the and an average running pace of 9:42 per kilometer, spendburning 1,104 kilocalories.

specifically for calories/kilocalories, we say "burn" usually

I was worried forabout suffering from hypothermia, so I wore two more wearlayers plus a raincoat.

for any illness or medical condition, we "suffer from X"
for clothes, if you mean number of jackets/pants/shirts or other clothes, we refer to them as "layers"

I thought I was going to be a "Cold Hot Dog" that was distasteful.

Not sure what you mean but make sure to have articles for countable nouns like Dog, it's still considered a noun even if it's in a nickname since you put "that" after it which makes it a noun being described

However, I became a "hot" dog again while running, so I took off the raincoat and a sport jacket.

article

However, ironically, we were surrounded by the deep fog, and I couldn't see any beautiful scenery there, and I hardly saw any runners ahead of me, with the fear of potential fear of getting ly going off-course-out and lost in the mountain.

there is no fog we are currently talking about or referencing so using "the" is not correct in this case. you have choices, the nuance is different:
"we were surrounded by deep fog" just means you were surrounded
"we were surrounded by a deep fog" there is an extra focus on the fog as a single entity, like you're talking about the entire fog as you say this

"i hardly saw ANY runners" need to include the any for stuff you don't see or don't have. you got this correct with the scenery, it's okay to say it twice :)

correct term is off-course meaning to stay away from the path or get lost. no such thing as "course-out."
we "go off-course" not "get off-course" (confusing yes i'm sure, it's okay, it's one of the harder things about English to really know by heart all of the proper "pairs")

your fear is of "potentially going off-course." potentially means "maybe" or "a chance of." so it's not correct to say you have a "potential fear of going off-course" since you are afraid of going off-course. a "potential fear" means you are "maybe afraid"

I didn't know if it was actually foggy, or if I was just in the misty rain.

The uphills and downhills were so steep, very wet, and slippery that I had to walk instead of run.

Originally I started running for my health, but if I fell to diemy death or got severely injured, my attempt would have turned out in vain completely.

"to fall to [possessive pronoun e.g. my, his, her, their, your] death" is a stock phrase in English, it's like a whole verb by itself. being familiar with these is one of the big steps to becoming very fluent, you're on your way there ;)

you need to say "would have turned out" because it did not happen yet, it's a hypothetical situation.

no need for the completely, "in vain" already means it was for nothing

I knew well from my recent actual experience that I tended to fall quite easily, so I knew I musthad to be very careful.

i "tend' to fall = present tense since you're talking about a personality trait or habit that you currently have up until now
i "had to be" very careful because you were talking about being careful in the past, but it's already done. "i must be very careful" means it's happening now.

Yesterday, I ran 10k in the mountain in an official trail run race in 1:42:36, with thean average heart rate of 151 bpm, with the average running pace 9:42 minutes per kilometer, spending 1,104 kilocalories.

151 bpm? Wow, you must be in great shape.

I was worried forabout suffering hypothermia, so I wore two more wearlayers of clothing plus a raincoat.

I thought I was going to beturn into a "Cold Hot Dog," thawhich I thought was distasteful.

The scenery of mountainsmountain scenery would have been terrific beyond my imagination ifhad there had been no fog.

However, ironically, we were surrounded by the deepick fog, and I couldn't see any of the beautiful scenery there, and I hardly saw runners ahead of me, with the potential fear. It was easy to lose sight of the runners ahead of me. Besides, I was worried that I would run off getting course-out andthe course and end up getting lost in the mountain.

I did some rewriting to improve readability:

However, ironically, we were surrounded by thick fog, and I couldn't see any of the beautiful scenery there. It was easy to lose sight of the runners ahead of me. Besides, I was worried that I would run off the course and end up getting lost in the mountain.

I didn't know if it was actually foggy, or I was in the misty raina drizzle, or if I was in a cloud because of the high altitude.

Or I was just in the clouds because the altitude of that place was very high, but not that high as I noticed thin airEven though it is a high mountain, I didn't feel that the air was thin.

Even though it is a high mountain, I didn't feel that the air was thin.

The uphills and downhills were so steep, very wet, and slippery that I had to walk instead of running.

Originally I started running for my health, but if I fell tohad fallen, died or got(ten) severely injured, my attempt turned out in vaiwould have been completely in vain.

I knew well from my recent actual experience that I tended to fall quite easily, so I knew I must be very careful.