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Keep cycling all the time. On day 1 last year I was constantly getting overtaken by other riders only to then find them in the next town buying stuff.
After 2016 TCR, when I couldn't think about anything else other than TCR, I did analysis of how long people had spent riding per day vs stopped. I looked at all the people I knew and you were the only one who had less stopped time than I had.
That seems to be definitely enough training. As @skinny said I really didn't do much training - about <1000km in the 12 months leading up to both TCRs. I had done some long rides, e.g. a few 400km + but that is mainly confidence building, in fact, those kind of long distances are almost certainly detrimental to your riding ability as you'll end up breaking down a lot of a muscle. If your aim is to do 320km per day and you sleep for 8 hours you only need to average 20km/h.
These are the things which I think are more important than training if your goal is to finish in time for the party.
Riding comfort - get the best bike fit you can afford, figure out which saddle, handlebar tape, etc. you like.
Pain relief - I carried tonnes of Ibuprofen, Paracetamol, Nurofen plus (it has codeine in and available over the counter in the UK), Chamois gel and some of that numbing spray. If I did it again I'd probably take some tape too.
Keep cycling all the time. On day 1 last year I was constantly getting overtaken by other riders only to then find them in the next town buying stuff. So make sure you have the capacity to take enough food and water for the entire day. Also, try and have your luggage set up so you don't have to stop, e.g. a frame bag for your essentials. I ended up doing everything whilst cycling including brushing my teeth, putting on extra layers and occasionally urinating if it was quiet.