Congratulations on the Miyata 912 selection. I would have recommended that, or the newer 914 version.

I just rebuilt my very old, but still good condition 912 with modern components (I know that is not your intention), and its nearly as nice a ride as my Rivendell Rambouillet. The only exception was the steeper seat tube on the 912, which made it impossible to get a Brooks leather seat back far enough for my very long legs. But that isn't a problem most would have.

For the 50+ mile rides you plan, expect to spend some time trying different saddles and their position. I solved my setback problem on the 912 with a leather Selle An-Atomica saddle, and already have a taller stem to get the bars up where I like them at this (less flexible) time of my life.

If your 912 doesn't have good brake pads (Kool Stop salmons are highly preferred) or dual-pivot brake calipers, I'd recommend both of those as worthwhile and less expensive upgrades for safety.