bikfor, can I assume that your question's relevance is vis-a-vis a fitness plan? If so, I am in the same boat as you. Back in 2010 that is all I could do as well, i.e., 10 mph average with lots of huffing and puffing and coasting on a flat course at that. I was prediabetic and needed an exercise program.

I would say that back then I was burning around 100 to 200 calories per hour. This is probably where you are now on your bike. Do not be disheartened. Just ride like you are doing now. Keep at it even when the temperatures drop, and in a year you'll be surprised at how you'll be able to continuously pedal for one hour without ever having to coast.

Also, each year you'll gain stamina, strength and become faster. I'm middle aged and started cycling only in 2010. I've gained about 1 to 1.5 mph each year. Nowadays I burn around 400 to 600 calories per hour, depending on effort for the day. I need to keep on cycling forever to prevent myself from becoming a fully diabetic person. Not a chore though, I've discovered that I do really enjoy road cycling. You and I, we'll never be fast racers. We just want to race against ourselves day after day. Hope this gives you encouragement. Good luck.