I ran the Fukuoka Marathon last Sunday and experienced leg muscle pain for two dayshe first two days (after the event/race).

For "leg muscle pain", this is perfect grammar but doesn't sound natural.

But a more natural way to say this would be:

→ "pain in my legs"
→ "cramp in my legs"
→ "pain in my leg muscles"

Or:
"I ran the Fukuoka Marathon last Sunday and had awful pain in my legs for the first two days after the race"

However, this morning I felt almost no pain ---just a little, if anything at all.

On the other hand, I’ve been suffering from both of my fifth toenails, whichpain in my toenails - both of my small toes have turned reddish in color.

For toes:

"Big toe" is the 1st toe
"Small toe / pinky toe" is the 5th toe

We don't use 1st, 2nd, etc. to count our toes.

I had the same experience with a toenail on my right toenailfoot ten months ago; it turned black and stayed that way for several months.

(Not 100% sure what you meant by "right toenail")

I’ll stop running for a few more days and see if the pain decreassubsides.

During the Fukuoka Marathon, I stopped at 11- or 12 water stations.

I didn’t come to a complete stop but slowed down at each station, which likely resulted in about five minutes of overall time loss overall.

I also went to the restroom seven times (forto pee) and stopped once forto poop, which took an additional ten minutes.

I actually didn’t feel the need to go that often., but I forced myself to go because of [XYZ]

You may need to explain this sentence as it doesn't feel like a complete idea.

I nehad targeted to lose ten kilograms before the race, but I only lost one kilogram.

Like this?? I am not clear what "I need" means in this sentence.

If I lost more nine kilograms, my time would have likely improved by around 27 minutes.

I could have cut down a couple of minutes for dropping byby using fewer water stations, and a further five minutes forom the restroom visits.

I could have cut down abroutnd 35 minutes in total.

Considering these factors, I could probably could have achieved my "sub-six" target (finishing within 6 hours by gross time).

I ran the Fukuoka Marathon last Sunday and experienced leg musclehad leg pain for two days.

Not incorrect, but I changed it to be more natural.

However, tThis morning I felt almost no pain ---—just a little, if anything.

On the other hand, I’ve been suffering from both of my fifth pinkytoe-nails, which have been hurting, and have turned reddish in color.

I had the same experience withthing happen to my right toenail ten months ago; it turned black and stayed that way for several months.

During the Fukuoka Marathon, I stopped at 11- or 12 water stations.

You could also say 11 to 12. I think the dash can be confusing if the numbers are consecutive.

I also went to the restroom seven times (forto pee) and stopped once forto poop, which took an additional ten minutes.

If I lost more ninnine more kilograms, my time would likely improve by 27 minutes.

I could cut down a couple of minutes for dropprom visiting by water stations, and five5 minutes forom using the restroom.

Hahaha.

Your entry so far has a clinical, slightly formal style so the Hahaha seems out of place