I aimed for "sub-six," which means that I would finish running within 6 hours.

I ran a practice run for 45 kilometers about 50 days ago, but it took me 6 hours and 50 minutes, which proved that I would not be able to run in "sub-six."

I lost my motivation and felt a "runner's low" instead of a "runner's high."

On the full marathon day, the temperature losewent up to 20 degrees Celsius which was hot for the runners and I got my legs became cramped after 30 kilometers.

or i would say "my legs cramped up"

I had to walk instead of running for the relast 10 kilometers.

However, after 17 days, I enjoyed running 6 kilometers, for uphill and downhill off-road running course yesterday.

Run wWith the Runner's High aAfter a While

I ran an official full Mmarathon race 17 days ago, which was torture.

While Marathon is a place in Greece, when talking about a marathon race we don't capitalize it.

I ran a practice run for 45 kilometers about 50 days ago, but it took me 6 hours and 50 minutes, which proved that I would not be able to run in "sub-six."

Grammatically this is fine, but I might add to the end something like "... without more training." to make it make more sense.

On the full marathon day, the temperature lrose up to 20 degrees Celsius which was hot for runners and I got my legs crampeds after 30 kilometers.

You can also say "my legs cramped/cramped up."

I had to walk instead of running for the restfinal 10 kilometers.

You can also say "the rest of the 10 kilometers." but it sounds slightly less proper. Totally fine in speech.

However, after 17 days after the marathon, I enjoyed running 6 kilometers, for on an uphill and downhill off-road running course yesterday.

My heart rate was able to rise to 158 beats per minute.

This is grammatically fine, but sounds strange out of context unless you have a heart condition. If that's not the case, then I would just say "My heart rate rose to 158 beats per minute."

I felt thea runner's high as wellgain.

This is technically correct, but for minimal confusion I wouldn't use "as well" here unless in one of these situations:

* Someone else had a runner's high, and you wanted to say that you had it too.
* There had previously been no mention of "runner's high", and you are now bringing up that it's something you feel while running.

I didn't remember it at first, but after seeing my self-introduction, I learned it was a wordplay with a "hot-dog."

On the day of the full marathon day, the temperature lose up toincreased 20 degrees Celsius which was hot for runners, and I got my legs cramped after only 30 kilometers.

I think you meant that it increased 20 degrees Celsius?

I had to walk instead of running for the relast 10 kilometers.

However, after 17 days, I enjoyed running 6 kilometers, for an uphill and downhill off-road running course yesterday.

I didn't remember it, but after seeing my self-introduction, I learned that it was a wordplay with a "hot-dog."

Running with the Runner's High after a While

I ran an official full Mmarathon race 17 days ago, which was torture.

I aimed for "sub-six," which meanst I would finish running within 6 hours.

I ran a practice run for 45 kilometers about 50 days ago, but it took me 6 hours and 50 minutes, which proved that I would not be able to run in"the race in “sub-six."

On the full marathon day, the temperature losewent up to 20 degrees Celsius, which was hot for runners, and I got my legs cramped after 30 kilometers.

I had to walk instead of running for the restmaining 10 kilometers.

However, after 17 days, I enjoyed running 6 kilometers, for an uphill and downhill off-road running course yesterday.

I aimed for "sub-six," which means I would finish runningthe race within 6 hours.

I ran a practice run for 45 -kilometers practice run about 50 days ago, but it took me 6 hours and 50 minutes, which proved that I would not be able to run in "sub-six."

I lost my motivation and felt the "runner's low" instead of the "runner's high."

On the full marathon day, the temperature lose up towas 20 degrees Celsius, which was hot for runners, and I got my legs crampeds after 30 kilometers.

I had to walk instead of running for the restfinal 10 kilometers.

I tried to run after that, but my "runner's low" continued, and I could only run for just a couple of kilometers after the race.

However, yesterday, after 17 days, I enjoyed runningasily managed to run 6 kilometers, foron an uphill and downhill off-road running course yesterday.

I moved "yesterday" from the end to the beginning of your sentence.

My heart rate was able to rirose to 158 beats per minute.

We don't usually say "my heart rate was able to rise" :-)

I didn't remember it, but after seeing my self-introduction, I learned it was a wordplay with a "hot-dog."

Thanks for explaining! I was scratching my head about it...