On the day...
65km is a long way and takes a long time to walk! 42km is a whole marathon and 35km can take up to 8 hours to walk.
Make sure you are fit enough to start training, and see a doctor if you have any doubt at all.
Start with short walks, depending on your level of ability and fitness. You might feel, for example, that you want to start with the equivalent of one lap of the walk – 7Km. That will take over an hour at a steady pace. Build up over time in the weeks preceding the walk. It would be good to get to the stage where you can walk 30-40Km 2-3 weeks before the walk, and then you should taper down your training so you are walking less in the week or two before the day of the walk so you can rest.