I didn't do any practice tours before I did the Trans America and I was fine. That said, I had been riding a lot locally and was experienced at other forms of self contained travel (backpacking, canoe camping, and so on).

If you are not experienced in packing for light self supported travel you will probably take way too much and maybe still not have all the things you actually need. My suggestion is to pack a lot lighter than you think you are inclined to and to adjust as you go by mailing things home and if necessary buying things you find you need or having them mailed to you from home via general delivery.

Given your lack of time on the bike, I'd suggest that you take it easy at the start and build on the daily mileage gradually starting with easy days in the beginning. Ten days to two weeks in you will be knocking out the miles and may even be ahead of where you would be if you pushed too hard at the start. My personal approach is to take it easy enough that I can ride every day and not need rest days.