Awesome story. 15mph is respectable over 18 miles. Just keep riding and hold off for a few hundred more miles before you start buying/replacing stuff like saddles and gears and entire bikes. Your body is adapting and will adapt to positions and efforts you'd never think possible. The weights coming off, water only on the bike while riding less than 20 miles, you'll put the hurt on your son when you ride together next. 😉
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I don't do: disks, tubeless, e-shifting, or bead head nymphs.