I have known about the Great Wall of China since I was young, but I didn’t know themany details about ithe wall, such as: How many was; its length?. I have known about the Great Wall of China since I was young, but I didn’t know many details about the wall, such as; its length.

you could also say; I have been aware of the Great Wall of China since childhood, but lacked knowledge regarding its specifics like its length. (to sound more professional) I've also replaced 'The Great wall' with 'The Great Wall of China' for more clarity.

I have known about the Great Wall since I was young, but I didn’t know the details about it, such as: How many, what was its length? I have known about the Great Wall since I was young, but I didn’t know the details about it, such as, what was its length?

I have known about the Great Wall since I was young, but I didn’t know the / any details about it, such as: How manylong was it? / What was its length? I have known about the Great Wall since I was young, but I didn’t know the / any details about it, such as: How long was it? / What was its length?

When I traveled to the Shanhai Pass yesterdaylast afternoon, I was lucky to know the aforementioned questions. When I traveled to the Shanhai Pass last afternoon, I was lucky to know the aforementioned questions.

When I traveled to the Shanghai Pass yesterday in the afternoon, I was lucky to know the aforementionedfind answers to these questions. When I traveled to the Shanghai Pass yesterday in the afternoon, I was lucky to find answers to these questions.

When I traveled to the Shanhai Pass yesterday afternoon, I was lucky to knowlearn the answers to the aforementioned questions. When I traveled to the Shanhai Pass yesterday afternoon, I was lucky to learn the answers to the aforementioned questions.

Your original wording means you were lucky to know the QUESTIONS (but not the answers).

The Great Wall is a little more than 21,000 kilometers. The Great Wall is a little more than 21,000 kilometers.

The Great Wall is a little more than 21,000 kilometers (long). The Great Wall is a little more than 21,000 kilometers (long).

It’s more natural with “long.”

It starts from the east part of China and ends in the west part of China. It starts from the east part of China and ends in the west part of China.

You can shorten the sentence by using 'ends in the west.' instead of ' ends in the west part of China.' to shorten the sentence but it doesn't necessarily mean that it's wrong.

It starts from the east part of China and ends in the west part of China. It starts from the east part of China and ends in the west.

It starts [from | in] the east part of China and ends in the west (part of China). It starts [from | in] the east part of China and ends in the west (part of China).

“In” is more natural than “from” in this context.

TheIt’s start point of it ising point, called Laolongtou, which sticks into the Bo Sea near Shanhai Pass District. It’s starting point, called Laolongtou, sticks into the Bo Sea near Shanhai Pass District.

During this trip, I climbed the Badaling Great Wall in Beijing during this trip. During this trip, I climbed the Badaling Great Wall in Beijing.

I was shocked when I imagined how hard it would behave been to build the Great Wall in ancient times. I was shocked when I imagined how hard it would have been to build the Great Wall in ancient times.

I was shocked when I imagined how hard it wouldmust have been to build the Great Wall in ancient times. I was shocked when I imagined how hard it must have been to build the Great Wall in ancient times.