Thanks for the great welcome and advice, everyone! I didn't realize helmets weren't made to take more than one blow. That's good to know. Safety first. (Although hopefully there won't be any more crashes or spills!) I've decided to go around the speed bumps until I'm more sure of myself.

My bike is a single-speed. My town is fairly flat. However, even gentle inclines in my apartment complex parking lot have me huffing and puffing. Today I ventured onto the street for the first time and biked to the store, which is about a mile away. I had to stop periodically and walk the bike. I do a lot of walking around at work, so I think I should have been able to ride a mile on fairly flat road without stopping. Not sure if it's me or the bike. Since I don't know how long it sat at the big box store before I bought it, or if the good folks who put it together knew what they were doing, I picked up some dry teflon bike lube and will apply it to the chain tonight.

If lubing the chain doesn't help, then I'll just have to make do with the bike I have until I either gain strength, or until I'm able to upgrade to a bike with gears. Whichever comes first!