Congrats on your bike purchase and hats off to your commute on that bike! You are a rare one who rides a bike like that that far. IF you keep your caloric intake in check, you'll burn through that 15 lbs. in short order.
A few thoughts from a guy who rides beach cruisers long distances (40-100+ miles), though as stated earlier it's a very personal thing.
1) Get some bicycling shorts- you'll be glad you did. The slight padding helps a little, but their primary job is wicking moisture away from your body. You'll find a huge increase in comfort especially at those distances and effort required for that bike. I prefer the standard short type ones, not the smooth hip hugging hardcore biker look. Wicking t-shirts are also more comfortable than standard cotton.
2) Over the several bikes I own I prefer a slight forward posture especially for higher speeds and longer rides on my 3G Isla Vista. The swooped back bars are great for slow speed cruising (it is a cruiser afterall) but if you're battling winds or climbing hills you'll tend to lean forward or stand up and it's near impossible to peddle the bike comfortably with your arms beside or behind you so the Isla Vista shortened swooped bars seems to be perfect for my style of riding. I really notice a difference when I ride a bike with bars that are longer.
3) At some point that seat might need to be changed. Don't scrimp on the seat as it's the most important thing to your comfort. In the beginning I bought a $50 seat for a $25 bike because I knew if I hated the seat I wouldn't ride the bike. It worked, I'm still riding bikes 3+ years and almost 14,000 miles.
4) On longer rides on a bike like that, tire pressures are critical, more so for comfort as in making the overall journey more enjoyable. Make sure they are maxed out (I often run a little over) and check them weekly. Keep the chained lubed well.
5) Hand grips at some point. Keeps your hands from getting numb which isn't common on a cruiser but at that low of a price point keep in mind you may need a new set sooner than you think.
Enjoy the bike and the commutes to work, they're a great eye opener in the morning and relaxing in the evenings.