drop vs. flat bars
Lots of good advice here! Wait until you're 62 and dealing with creaky wrists and elbows! That said, the drop bars should be more comfortable overall than flats because you have more hand positions. All things being equal, you should have been able to do your 45 miles (perhaps a bit much first time out on a new machine) with your hands atop the bars and feel the same comfort level as with flat bars -- don't you think? You might want to consider 1) raising the bar and 2) whether you should be using a shorter stem and even a riser stem. Have you measured the length of the top tube and stem to see if you now have a longer dimension in this regard than your Bianchi? Some stretching and strengthening of neck and back muscles will help, as will shorter rides for the first few weeks with the new bars, but do look closely at the above possibilities. Probably, one is extremely lucky if fit is spot on at the first attempt.