I did a similar return to cycling a few years ago. My advice is this...
1. Staying in shape and training is always good but huge amounts of training are not a prerequisite for a long tour. It does help greatly to have enough time on the bike to be comfortable for longish days in the saddle, but being a trained athlete is not a requirement. The key is to set daily goals that are reasonable and achievable. It is bad news to attack a trip and find yourself worn out 2 or 3 days in. Much better to ease into the trip, hitting your stride in a week to ten days.
2. I am convinced that all the hype about the ultimate bike, racks, saddle, panniers, sleeping pad, or whatever does a disservice to new tourists. I did my first tour (Coast to coast on the TA) on mostly inexpensive gear and a $599 bike. I didn't regret any of my gear choices other than the fact that I started out with a bit too much gear. Some gear I have since upgraded, but a lot of stuff I actually found I prefer some of the cheap items I used. The bike itself is the last thing I think about when thinking about my tours.
3. General rule of thumb, if in doubt leave it home. Then when under way periodically, go over your stuff item by item and mail stuff you aren't using home.
4. Another rule of thumb, don't try to think of things that might be useful. That approach will add lots of items to your list that sound like a good idea but you really don't need.
5. Having someone at home that can accept things you mail home and send you things via general delivery you decide you need is a big help. For example there may be things you only need for one geographic region of the tour. Why carry them the whole way?