I didn't have wrist pain, but I did have the shoulder thing when I first started riding for more than maybe an hour. I think part of it was the stress of being new at it.

As has been said, make sure your bike fits and that core thing is so important, too. Let your midsection do a lot of the work and take some of the strain off of your wrists/hands/shoulders/neck.

You can totally do 150 in 2 days. You have tons of time to work up to long days of pedaling. I did my first century after ~8 months of riding. I was no speed demon, but I never regretted for a minute making the commitment to do it. And it was a really fun day. The next day we went for a 25 mile cool-down and my butt felt great!