I have a buddy who did silver 2 years ago and have done the roof myself (only bronze but that was a good6-7 hours of riding each day)
As mentioned fitness is key - best if you are able to do bike specific fitness. a few 2 day events will also help you deal with that aspect of getting up early, riding the whole day, followed by bike servicing and then doing it again the next day (bonus if you only have to do the riding and have a good second crew). This type of training will also help you get to grips with what you are happy eating mid ride and how best to recover for the next day.
Long days in the saddle will also help you know what kit is comfy and what is not, do you need underwear (tights for chafe etc), what needs taping and the best way to do it. It also helps you know how you react when you are exhausted and how empty the tank can really go (research navy seals 40% rule)
Nutrition is second key - cant ride on an empty stomach. I did a fair amount of reasearch on this and high-5 does a 4:1 energy drink that i used (they also have some good info if you want to back it up with science). basically when you are exercising for extended periods of time you need a high protein drink to fuel your muscles, combined with some caffeine as this helps to absorb what you are taking in to get digested and into your body (high5 do energy sachets with caffeine and some with out, i used both to give me a boost every now and again and especially after a pit stop that involved eating). when your body is highly stressed it starts to shut down unnecessary parts of the body - like your digestive system - this is why you need the caffeine to give it a boost.
The 4:1 can also be used as a recovery drink at the end of the day - you need to be hydrated for the next day and your muscles need to recover when you sleep. You may also find that you want one bladder with energy/hydration drink and another litre or so of plain water (this is what i did, when it got real hot a sip of water was the refreshment i needed mentally and also to energy drinks can be pretty sweet and not refreshing)
A year or two ago I did 6 months of Banting diet and swore that if I ever did an extreme enduro event again I would do it on the Banting diet, but then you need to be really disciplined and it cheat (modern thought from the country cling world is that you should cheat while you are on the bike). You body then adjusts to ketosis so you start to burn rather than store fat. I never experienced much difference in energy levels but did notice that I never got hungry while out riding and so I believe it works.
Fatigue: you will be doing this for 4-5 days. try to get your body clock set to romanian time before you arrive there (taking into account that you will be getting up super early each morning). you want to be able to go to bed and sleep and be ready to eat breakfast when you wake up.
Muscle recovery is important too so as mentioned a good recovery drink is good, also need to rehydrate (with the salts, not just water - after a long sweaty day of riding a salt tablet will taste as good as a worthers original lol).
An ice bath is great for recovery (if you stay in it long enough). And if there is an option for a proper massage take it (not the ones with the smelly oils or a happy ending - you need that protein lol). Poor mans option is to sleep in full compression skins (sport tights and top that help circulation)
Then the last is Skills.
If you can do a training day with a pro (who knows how to teach) do it. When you are tired and reactions are slow it is nice to be able to break it down into manageable steps (weight the peg, blip the throttle and clutch etc) - you don't have the enery to be doing multiple attempts at an obstacle, stop, plan your attack and make it count.
You are not racing so think about your line first - it takes a lot of energy to pick a bike up from the bottom of one of those mountains. (i know for a fact that i finished the roof because i didn't have to pick my bike up - i had an 11 month old baby at home, combined with doing the fitness training and bike prep i was not as fit as i would have liked to have been). <PS some guys i know have included 100 bike pick ups and put downs into their weekly training schedule>
Then there is also romaniacs specific training (cant help too much here) believe there is a lot of pivot turns, riding across a slope, and riding over logs. (but everyone i know who has done it says the riding is awesome!!)
Another tip for those super long climbs is to set small goals, and preferably make it a point that is easy to start from. Don't worry about the top of the climb, just worry about the next 20m you will be surprised how you still get to the top!
All the best, i'm suuuuuuper jealous!!