How hard is it to get to 1800 FIDE?

My Playchess rating right now is 1739. My rating on chess.com is under rated because I play the 5|5 pool(to increase my playing strength).

How hard will it be for me to get to 1800 FIDE?

1800 in over the board slow (40/90) chess?

What is the relevance of playing 5 5 blitz?

Yes, that is correct. 1800 in over the board Tournament play. I play 5 5 blitz to get used to slower time controls.

How old are you? What are your study and training methods? How do you analyse your games? Can you get help by a coach or a much better player?

Anyway, 5|5 is still blitz chess. You will be much too fast when you first play slow chess over the board. I would suggest to play some games with at least half of the time the first otb tournament will last.

And some people who plays 2D on the screen only getting vision problems on a real board. Keeping score, pressing the clock and the athmosphere in a hall full of chess players is much different from sitting in your living room.  Some online players have really great problems when they first play otb. Others are even better than online. So you have to check yourself.

Well actually I hvae already played FIDE rating tournaments. I have a current FIDE of 1345. I want to get to 1800.

So what was the time control of the FIDE over the board tournaments you played?

It was 90|30.

So in what way would 5|5 be considered relevant to the strength needed for a slow game? 

Try 45|45 at least. You have to get used to the kind of analysis you can do in your head when you have the time. If you are not used to concentrating for long periods, it takes practice.