210 miles / 15.5 hours = 13.6 mph for sunset finish of coast to coast (15 hrs, 30 minutes)
Per one online bike wattage estimator
that requires, assuming 230 lbs + 30 lbs of bike, riding on the hoods on gravel, approximately:
105 watts, pretending it's flat and has no sand
Internet sources suggest maybe 60% of FTP for double century type distances if not super fit.
105 / 0.6 = 175 watts
or more conservatively
105 / 0.5 = 210 watts
which gives a one hour flat asphalt slick tire pace needed of between 18.8 and 20.3 mph.
So some ground to gain before readiness, since with the odd lights and a few small hills, I'm starting at about 16 mph for an hour (120 watts), which would only be between 10-11 mph, way outside of the sunset box, and flirting with the entire cut off of 21 hours.
What about if you stopped being fat, Jens? Dropping 50 pounds takes the 105 watts to about 92 watts, again assuming it's flat. So yes, 10% for free, so not to be dismissed. (Plus the associated hiking hill climbing benefits, etc.)