That looks like a nice bike for path riding. The stem appears to be adjustable, maybe lowering it a little would put your arms in a better position to control the front end and quell the wandering? It could just be you getting used to cycling, the balance and control should come with a bit more time on the bike. It sounds like it fits well; take them up on their 'tune up' offer and mention anything you noticed that you think needs to be addressed. And yes, maybe starting with shorter rides and building up would be a better way to go at first?

I agree with the others here who probably know a lot more about nutrition than me as I am a junk food addict in remission (sorta). Congrats on the weight loss!

I know I need to have some carbs before I ride; oatmeal or granola in the morning, and a well rounded dinner the night before. When I bonk it's like I fell off a cliff. So I take water, some electrolyte drink if it is hot (or electrolyte chews in the winter) and a granola/energy bar or banana with me on the ride. Also been working at staying hydrated all the time as opposed to just thinking about it on the bike. It helps.