You sound to me like you are much better prepared for the trip right now than many people will be at the start line.
Simply riding every day will give you enough saddle time to condition your butt. You dont NEED any training like spinning or weights, but stretching is always good. Get out and ride a few longer distances at weekends. Start with a 20-25 miler, then go for a 50. 50 miles is not that far, about 5 hours in the saddle at most.
Get some good kevlar belted, high pressure slick tyres (1.5" for an MTB wheel, 28-32mm for a hybrid wheel).
Do you use to clips or clipless pedals? On longer tours either of these will make cycling more efficient. Also make sure you can handle any common problems like punctures or snapped cables.
It helps to have some nice cycling kit, a couple of pairs of shorts and jersies, some good gloves ,helmet and shades.
You need a water bottle cage or 2. If your saddle is OK, then keep it, but if it gives you problems on longer rides , look around for a replacement long before the tour.