This will be my first 400km ride. Simultaneously really excited but also dreading it. No idea how my body's going to cope. Was okay(ish) on a 300, have since had a bike fit and hoping I can manage my eating a bit better so that I can finish.
Wouldn't worry about that. The temptation on a 200-300 is to go a bit too hard... typically on a 400 you will keep a much more relaxed pace. The only time limit that would make me go quicker is if I am in sight of Chalfont and it's not 11 PM yet... being able to rush to the pub for the last order would be a massive plus for me.
But that means being back in less than 17 hours, which is a tall order.
My feeling is that at LWL there will be some very fast guys, so I'll have to refrain from sticking to their wheel at 30 km/h... if I lose them early on, I'll just do my own thing and I know how to manage my pace and food intake. I recommend you do the same