it is/ there is

father93,
It is hot today.
It is wonderful to see you.
It is wrong to steal.
It is a big house!
It is 20 miles to London.

We use "it" sort of like a "dummy" subject a lot in English. There isn't a logical explanation, we just have to accept that as part of the language.

father93 I think.... There is ...is better..

No, you think wrong

In this case, you are using "it" as a pronoun. A pronoun renames other nouns, in this case, perhaps the word "distance." If I would re-write the sentence without "it," I would say "Father said that the distance is 20 miles (to London)." But instead of saying "distance," I will replace with "it."

ok. And for example :

It is a complete mystery how they ever got there in that car . Why IT...I think There would be much better

father93,
It is good to be inquisitive when learning something. But when that inquisitiveness turns into relentless questioning, it begins to work against you. If you insist on your own answer, fine. But you need to know, "There is" has the combined meaning of "have" which is not what you want.
There is a box of fresh donuts in the coffee room counter,

father93here in that car . Why IT...I think There would be much better

It (the dummy it) is the actual subject of the sentence in these examples:

It is raining.
It is hot outside.

On the other hand, there is not the subject in an inverted sentence. The real subject comes after the verb.
There