I feel like the 90-minute (+ 30s increment) games are just perfect for getting better, but it might be caused by the fact that I'm used to it. Pretty much any longer time control will do the job.
But definitely, your choice to play with longer time controls is a good one. For me, a 30-minute game is a rapid game and even FIDE thinks so :)
A ‘Rapidplay’ game is one where either all the moves must be completed in a fixed time of more than 10 minutes but less than 60 minutes for each player; or the time allotted plus 60 times any increment is of more than 10 minutes but less than 60 minutes for each player.
The big difference will obviously be the "redundant" time for you. I think you can use that for improving in some techniques like double-checking and evaluating positions.
The idea is to use these methods extensively (and consciously) until they become part of your unconsciousness. I learned this idea from a book called Pumped Up Your Rating by Axel Smith.
Some other advice:
- Your brain can adapt to the time quickly and, especially if you're a "time-trouble player", what might have seemed as a lot of time before is suddenly not that much... That doesn't mean you shouldn't think, just anyway be aware of the time situation on your clock.
- Don't be hasty when your opponent starts playing quickly. That applies generally, but in longer time controls it is more important than anywhere else. For example, you have no reason to become nervous when your opponent has 80 minutes left and you have "only" 55 minutes on your clock after 20 moves - it's quite a lot of time. Just stick to your time management.
I guess the last point falls into the category of mental preparation because you might feel pressure in such a case. So it's good to be prepared for that.
The last thing I have on my mind is how you spend the time when it's your opponent's turn. In rapid games, players are usually used to sit at the board throughout the entire game, which is almost impossible in case of long games.
It usually depends on the character of the player, some stand up from the games very often, some not. Just keep in mind that you shouldn't distract yourself too much when you're off the board as you don't want to build your concentration from the bottom-up again. I think you will have to try this yourself.