Comfort and reliability are key, don’t want to be stranded 50miles from no-where-ville with no cell service.
I never intended to become a retro-grouch, but here we are:
A good aluminum rim with 24-28 spokes is more comfortable than super stiff deep-V carbon wheels.
Consider carefully tubeless. You may wind up with a few more flats with an inner tube, but you can repair flats road side in 10 mins without (too much) swearing or getting sealant all over yourself.
Electronic shifting is awesome, but the ‘distance’ bike is mechanical shifting, if the electrical shifting dies you’re stuck in a single gear. With mechanical shifting you will likely be able fix roadside to continue riding.
I prefer somewhat wider handle bars (1-2 cm), set a little higher compared to purebred racing bike, a little bit lower bottom bracket, but still like responsive handling so my distance geometry is more or less classic European road bike for head tube and seat tube angles.
Before getting a bike fit take some really long rides. I never had any problems up to 100 miles, but when I started doing double-centuries and longer the inside of my left knee would start hurting at about mile 150. Knowing how you feel after many hours on your bike will help your get the most out of your bike fit.