Back pain could indicate a bike fit issue. Many bike shops are willing to take a subjective look at your bike position and give you suggestions. A few shops will even professionally fit you to your bike for a fee.
As far as cold weather riding you can buy bicycle specific clothing or many other less expensive options may work. Crosscountry skiing gloves can work well. I'd get a very thin hat or baklava to wear under your helmet. Light weight fleece can be very helpful.
Snacks are good I usually eat a little something every 30 to 40 miles or so. Everyone's calorie needs are different.
I want to encourage you to keep at the fitness. You'll continue to make gains as you train appropriately 👍