Ditto everything said above about keeping elbows loose, down and in! You'll find it makes a big difference! And let yourself sit upright every so often as you're moving along, to give yourself a chance to stretch your neck and shoulders a bit.

On the shorts front, if at the moment you don't feel comfortable with regular bike shorts, how 'bout trying something like the Shebeest Shindiggers? (Or another just-below-the-knee capri.) I much prefer the Shindiggers to standard shorts because they're not as tight AND don't have the grippy elastic stuff that makes you feel like you're wearing sausage casing. They come down just a bit over the knee and (I think) look slenderizing. I wore them on a multi-day bike tour recently and loved them. Not too warm and no shorts-length tan line!! The chamois padding isn't thick, but that's my preference anyway........

Have fun!!