Welcome to TE, fellow Giant owner! (And yes, bikes are like potato chips (crisps?)--can't have just one. My bank account hates me.)

Yes, core work is very important. I can tell when I've neglected my core. (Like lately, for instance...) I do remember that death grip, and core work and putting in time on the bike helps. You might want to get a bike fitting and check the saddle threads over in the Gear and Accessories section to rule out saddle or fit issues, as those can force you to put weight on your hands and make it so you have a hard time taking your hands off the bars.

As for the muscular thighs/butt thing: It depends on your individual physiology and how much you ride. I haven't lost a ton of weight, but I've definitely lost fat and gained muscle mass. I definitely don't "bulk" as much as some people do, though.

The important things are to make sure that you're comfortable on the bike and to have lots of fun. And don't burn yourself out.