I was the same. Almost exclusively 3/0 for years before going to a tournament.
I don't know if this true for everyone, but it really hurt my time management... I played too slowly! Since I'd never done much analysis at all, I didn't really know how, so I would calculate around in circles sort of aimlessly, over and over.
Which is probably the biggest difference. In long chess you do analysis. By that I mean you calculate a line for at least 2 different candidate moves. You form an evaluation for the end positions, then you compare the two and choose the more favorable one. In blitz it's mostly short bursts of forcing moves and intuition... and if you do make a long calculation you're basically forced to play it because you've invested too much time to consider other options.
Also in blitz the value of initiative is higher and the value of technical endgames is lower. In a blitz game you might get a win or drawn endgame, but it takes too much time or precision, so you draw instead of win, or lose instead of draw. Therefore middlegame strategy in general is a bit different.
There are also practical things that don't exist in blitz. I was talking about this a few days ago... lets say you have 30 minutes vs your opponent's 15 and they're defending an uncomfortable position. If their position has long term problems, a good way to use this time advantage is to avoid transitioning to later stages of the game (piece trades, pawn structure changes, etc). Keep your ideas alive, but don't execute them. 20 minutes later the clocks will be approximately your 20 minutes vs their 5. This is when you go into transitions, and give them problems to solve.
So definitely you shouldn't be ashamed to lose to an 1800 at the tourney. But even if I hadn't checked on your history, I would have said the same. 
Some of this may be obvious, but you say it's your first, so I'm just making sure.
Handshakes before and after the game. You don't have to say good luck or good game or anything, but some people do.
When the game is over, it's always tempting to start analyzing or at least make a few comments right away. Most people make some brief comments, but proper is to leave the playing hall so you don't disturb other games. I'll usually simply say, "do you want to look at it?" and if they say yes we move to the skittles area to do the post mortem.
You don't say check. In fact, if you're being proper, you never speak to your opponent at all during the game except to offer/accept/decline a draw, and never when their clock is ticking. If there's an issue like an illegal move, or your opponent is being distracting (both very rare), the proper procedure is to stop the clock, get up, and find the tournament director to make your claim / complaint.
Similarly it's not proper to adjust pieces on your opponent's time. Let's say a piece is touching the edge of the square and you want to move it to the center, you shouldn't interrupt their calculation on their time to do so. When it's your turn you say "I adjust" or the traditional "j'adoube" and center the piece. Tournaments are played with touch move rules meaning if you touch a piece you have to either move it or capture it (if it's legal). By saying "I adjust" you can temporarily bypass this rule.
If you're going to withdraw or take a bye (meaning not play for a round), let the tournament director know as soon as possible. That way your potential opponent wont be left with no one to play when you suddenly leave.