As with any marathon getting your pace right at the start is crucial and ensuring you go off at a speed that you will be able to maintain for 26 miles. Go off to quick and you’ll hit the wall.
Getting your fuelling correct is also key with 30-60g of carbs needed for each hour you run. Also if the weather is warmer in Barcelona than you are used to training in then make sure you consider a slight alteration to your hydration strategy.
This isn’t a race I have much knowledge on, but I hope the links will help. It appears in 2024 there will be a new route many there are fewer turns involved and less elevation, both very good news for runners looking for a fast time.