Are you training indoors in the same position that you’re racing in? If so the muscular endurance needed to sustain that position shouldn’t be an issue unless the race has a load of climbing where the slope can emphasise different muscles. If not, then you need to get that TT bike on the trainer. I know a lot of people who do 90% of their training in a road position and then wonder why they struggle to hold a TT position on race day…
Figuring out hydration can be tricky if you do most of your training indoors since sweat rates outdoors can be very different. I suspect over a 3 hour race at steady power that hydration and electrolytes is more likely to be an issue than getting enough calories. Not wanting to get too graphic, but simplest measure for me is how much I need to drink after a race before needing to pee, and how much until said pee is relatively clear. If it takes 2-3 litres of water before producing a dark yellow pee then I clearly needed to drink a lot more…
Agree with others above that seeing a physio to check out leg length, strength and flexibility discrepancies is your best bet, particularly with your injury history.