Tried with AMG GT and oh boy, is this a crazy challenge. Was running mid 58s and the AI was having a comfortable 7 second lead. Made a mistake and quit on lap 7. Back to the drawing board for me.
Yah, if 58s are max pace while pushing 95%+ it's very difficult to maintain for an hour -- lots of places on this track where it's easy to lose time with a tiny error. If you are at the limit looking at track map or fuel mix or anything can distract you enough to lose it. lol. Even stopping for a pit can get you out of rhythm if you are that close to the limit. If you can manage 58s driving 80-85% and even a dab of fuel saving it's not so bad and then you can dip into 57s if you really need to during race for short periods.
Maybe practice outside of the event until you can more consistently get the lap times you desire where you don't feel you are on the edge for an hour. It's less frustrating to do this in TT mode where the pressure is off and you can use a ghost to try out some different lines and see impact. Shaving 0.25 off a couple corners can accumulate to a lot over a 30 lap race and when you commit it to muscle memory less likely to mess it up. Even if the car isn't exactly as BOP in TT mode you can set ECU to match BOP HP and it's close enough that for your lines it won't make a difference. You'll be faster with constant good tires and no fuel weight in TT, so you probably need to hit 56s and consistently 57s in TT mode to translate to 57/58 in race when tires are good. Improving lines tend to also save tires so you'll see less degradation.
I went through this process and went from 2nd place back about 12 seconds to a win by 2 seconds (and could have been 8-10 seconds but bad luck with traffic). I watched a video and found out I wasn't using enough curbing on turn 2... went from losing time to AI there to gaining a tad every lap. Same with turn 11 hairpin were you can cut well across curbing and get on throttle a lot earlier that way for the long full throttle section to follow. I was already good on the S's -- that's where you can gain lots of time on the AI and it's also where luck plays a major part -- hit traffic in the middle of this and you get slowed a lot losing time versus gaining time with clean run. You need to get the entry nailed in the S's which ultimately leads to high exit speed from section and you can gain chunks every lap on AI here.
Another things is learning best overtaking spots... if you can get ahead before S's by all means dive bomb... you don't want to follow there if at all possible... Other parts of the tracks there isn't as much of a rush. Probably my biggest weakness was taking too much time getting passes and being held up more than I should have. Can't practice that in TT mode! The AI are like trains... they can't be bumped out of the way, but you can late brake them in a few places and though they try to fight back you can make it stick if you don't completely mess up exit.
After a good long session in TT mode I went back to race and improved a lot. Besides getting some 57s in race the drop off over 10 laps stint was less pronounced... no more dead tires at end causing me to lose chunks of time just prior to pits. Good luck, don't give up. maybe take a break from race pressure and practice getting consistently and comfortably fast[er] first.
PS> if I were going to work on this more [I'm not, win is a win and no more credits to earn] I would work on the final corner exit and trying to perfect that. If you are following here and can make a pass down the long straight that will save you a lot. I would match exit speed of AI here, but only once did I manage a draft pass down the straight. I'm pretty sure the better drivers could have come out with better speed and made easier passes that don't compromise time like they do on most of the rest of the track -- not to mention maybe gain you time with closer/faster drafting down the first half of straight.