get fitted properly by an experienced bike fitter.
many riders using old information end up on a frame 1 size too large for their ideal setup.
get the smallest frame which fits properly, will allow fitting in "both directions" (shrinking or expanding) by adjusting stem, seatpost, saddle, pedal cleats, crank length, bars, bar taping, control setup, etc.
look carefully at your drop and reach. saddle height is critical, many run it too high..
personally? I ride a 54cm race bike (Specialized Tarmac) but with less drop and less reach than would be normal, it fits me perfectly now!
had a fitting session which made fundamental differences to my setup; something that came out of that session was difference in my leg lengths which is very common for many riders, and causes instability / pain / injury unless compensated for during the fit.
the fit also addressed the right shoulder pain I can get from repeated dislocations during years of freeride mountain biking and other extreme sports
"good fit" is one of those things that I would not suffer from doing 20-30 miles locally on a Saturday morning ride
but yesterday doing 104 mile ride solo in strong winds from Milton Keynes back to London through Bucks/Northants/Herts; good fit makes a massive difference between completing the ride and baling out to take a train home, 1/2 way through the ride
wheels and tires very important, I run handbuilts (Hope hubs on Mavic Open Pro rims with d/b spokes and Specialized Roubaix tires), quick light but also comfortable and shrug off potholes impacts!
quality clothing like comfortable bib shorts (with chamois cream), fitted shoes with proper footbeds and shims, merino wool socks, overshoes if cold, knee warmers, good gloves, upper layers, correct hydration, energy foods, some nice coffee whilst taking the train before the ride, all make a big difference for the long miles