First let me say that I wish you all the best with your proposed trip. It sounds like a big challenge and will be a great experience.

You may find that you actually don't mind carrying a load on the heavy side, but...
I strongly advise that you try your best to keep the weight down. On our coast to coast trip we sent stuff home several times. Sending 2-4 pounds home made a noticeable difference in the mountains. Riding alone the difference will be less noticeable because you don't have other riders to keep up with or measure your progress against, but still it does make a big difference.

You really don't need all that much crossing the US so personally I would start with 30-40 pounds of stuff max. If when you get into a part of the trip that you feel you need more, add more stuff as needed. No need to train with the big load, after riding the first few thousand miles you will be in great shape whether you started with the big load or not.

If you have someone at home it works great to mail stuff back and forth. Send home anything you don't use or can do without. As climate changes, have warmer/colder gear sent from home. This will definitely work for the parts of your trip in the US and Canada. After that you will have a lot of miles under your belt and will certainly know what will work best for you.