Welcome to the forums!
If your sit bones don't start to feel better in the next week or so, you may have a saddle that's "not right for you." If so, head back to the shop where you bought the bike (if you bought used or on-line, find a nice local shop). Talk to a salesperson about saddles. (Not to be sexist, but you may fare better with a saleswoman, simply because she may be more familiar with saddle-fitting issues for women.)
As for building mileage, just keep riding until you're tired. One trick is, if your legs are tired, shift to a lower gear and pedal faster (this shifts the load from your legs to your cardiovascular system); if your lungs are burning and your heart's a-thumping, shift to a higher gear and pedal slower (this shifts the load from your cardiovascular system to your legs). When both wear out, you're done.
Something that helped me put in lots more miles was to start riding with groups. I'm lucky in that there are a number of local cycling clubs, many of which have enough members that they can support rides at lots of different levels of difficulty. On my own, I'd do 16 miles and be done -- couldn't imagine riding further. First group ride, we went 24 miles and, seriously, it felt like half that. Now, group rides aren't time efficient, but they sure make the miles fly by. YMMV… some folks ride specifically to have time to themselves, and the idea of riding at anything other than their own preferred pace is a nightmare.
Oh, and while you WILL lose weight, you'll even more notice firming up as you lose fat and gain muscle -- so don't get too hung up on the numbers on your scale… enjoy how you start to look and feel!