What I have decided to do regularly is running.
Because I've been told by almost everybody that I gained a weight.
Actually, it's clearly obvious in both ways, : my body weight and my appearance, but I turned my eye blind a blind eye.
And I know what causes me to be chubby. There are two factors.

First of all, it's my lifestyle.
Over the past 3 months, I had to go and stay in several prefectures of Tokai area in Japan for a business trip, and the place where I had been staying was a business hotel, which means  meant there's  there was no way to cook by for myself. So I kept eating something that doesn't cost me a lot and easy  inexpensive, convenience food, which narrows  narrowed down my choices. the choice of the venue. Mostly these three choices, : Ramen, fast-food, and convenience store stores.

The second factor I guess why I gained a weight is might be due to age.
I am 27 years old now, and believe it or not, I used to be a kind of skinny guy. Even though I eat ate and drink drank a lot, that didn't affect my body.
But I clearly remember that after I turned to 25, my body, especially my tummy, started looking different.
This universal thing painfully made me understand that it's quite impossible to fight against age with no efforts.
If you want to maintain a healthy appearance your appearance nice and healthy, it's inevitable to keep making an effort.

As you might guess, this is my determination to keep going on a diet.
If you are or were on the same boat, and you happen to have some successful methods of diet, please let me know your approach. how to.

What I decided to do regularly is running. 

Because I've been told by almost everybody that I've gained some weight.
Actually it's clearly obvious in both ways, my body weight and my appearance, but I turned a blind eye blind.
And I know what causes me to be chubby. There are two factors.

First of all, it's my lifestyle.
Over the past 3 months, I had to go and stay in several prefectures of the Tokai area in Japan for a business trip, and the place where I had been staying was a business hotel, which means there's no way to cook for myself. So I kept eating something that doesn't cost me a lot and is easy to make, which narrows down the choices of the venue. ("of the venue" is correct, but it sounds a little artificial here.) Mostly these three choices: Ramen, fast-food and convenience stores.

The second factor I guess why I gained some weight is age.
I am 27 years old now, and believe it or not, I used to be a kind of skinny guy. Even though I eat and drink a lot, it didn't affect my body.
But I clearly remember that after I turned to 25, my body, especially my tummy, started looking different.
This universal thing (或は occurrence) painfully made me understand that it's quite impossible to fight against aging without effort.
If you want to maintain a nice and healthy appearance nice and healthy, it's inevitable to keep making an effort. (或は you have to keep making an effort.)

As you might guess, this is my justification to keep going on a diet. (或は stay on a diet)
If you are or were in the same boat and you happen to have some successful methods of diet, please let me know them.

英語はほとんど間違いないですね。

In my case, I successfully lost a lot weight, by simply eating a little bit less, and exercising a little bit more.  My weight loss was very gradual, but successful.  The most important thing is to weigh yourself every day and be aware of how you feel.

What I decided to do regularly

What I decided to do regularly is running. This is okay, but it would be more natural to write

I've decided to go running regularly.

Because I've been told by almost everybody that I gained a weight.
Actually it's clearly obvious in both ways, my body weight my weight and my appearance, but I turned my eye blind.  a blind eye.
And I know what causes me to be chubby. There are two factors.

First of all, it's my lifestyle.
Over the past 3 months, I had to go and stay in several prefectures of the Tokai area in Japan for a business trip, and the place where I had been staying was a business hotel, which means there's no way to cook by myself. So I kept eating something that doesn't things that didn't cost me a lot and were easy, which narrowsed down the choice of the venue. Mostly these there were three choices, Ramen, fast-food and food from a convenience store.

The second factor I guess why I gained a weight is age.
I am 27 years old now, and believe it or not, I used to be a kind of kind of a skinny guy. Even though I eat and drink a lot, that didn't affect my body.
But I clearly remember that after I turned to 25, my body, especially my tummy, started looking different.
This universal thing painfully made me understand that it's quite impossible to fight against age with no efforts.
If you want to maintain your appearance nice and nice appearance and remain healthy, it's inevitable to keep making an effort. you'll inevitably have to keep making efforts.

As you might guess, this is my determination to keep going on a diet. gives me the determination to go on a diet.
If you are or were on in the same boat and you happen to have some successful methods of diet know of any successful methods of dieting, please let me know how to.

This is a well written essay. Most of my corrections were an attempt to make your English sound more natural. 27 seems awfully young for your body to be changing so much--the bad stuff usually starts around 35. I bet you'll be fine if you don't eat junk food, do a moderate amount of exercise, and make sure to get enough sleep.