Just my humble input & encouragement about your concerns:
saddle sore - Your butt will get used to it fairly quickly.
bike breaking down - Don't skimp with your bike. Make sure it is in very good shape with all potentially old parts replaced.
hills!!! - Ha ha ha ha ha. If you go through Colorado, I can assure you hills will not be your worry. It'll be the BIG ASS MOUNTAINS that will torture you!
my stuff getting stolen while buying groceries or something... - Use a good lock. Some large supermarkets supposedly allow you to bring your bike inside. Always carry your valuable belongings on you when you leave the bike.