I'm just getting started and would love some input from both beginners and seasoned riders. I've been riding on a borrowed Pinarello for two weeks or so. I'm mid 40's been athletic for most of my adult life, with a few periods of inactivity. I think I'm already hooked! Gone on a few fun rides and up to riding 20 to 30 miles per ride. Here's are a few questions:
1. Is it normal for the base of my neck and top of my shoulders to ache so much? I've been riding some decent distances, but need to stop not because I'm tired but because my neck and upper back are screaming. Is this just a strength building issue or posture on bike or wrong bike?
2. My legs feel a little sore but in a good way...which I expect...but, my butt is really sore! Will this abate, do seats really make a big difference and is there a difference between men's bike shorts and women's...I raided my husband's cycling drawer and am wearing a pair of his.
3. I'm going to have to give up my borrowed bike and get one of my own...any brands/types that you love?

Thanks!