I'd agree with the suggestions to see a doctor. Be specific about your fitness goals and symptoms. Not just a routine checkup, otherwise you'll get the treatment other folks described: "You seem okay for your age."
Assuming there are no health complications, the only way to get stronger, faster or whatever your fitness goals are is to push harder. There are all kinds of fitness plans and tutorials online. Check 'em out and take a methodical approach, suited to your age, current condition and goals.
Having been sedentary as you described, it's going to take longer to improve. And it will hurt at times. There's no way around that. Muscles will burn, including the sensation of the lungs burning -- that's mostly the intercostal muscles in the ribs and diaphragm working harder than they're accustomed to.
I was very fit as a young man, including bike racing. But I was sedentary for almost 15 years after a car wreck broke my neck and back. I walked with a cane until 2014. I did as much walking as I could, but even walking up to five miles a day didn't prepare me for riding a bike again.
When I resumed cycling in summer 2015 I had to stop about every 400 yards to catch my breath. It took weeks until I could ride 3 miles, mostly downhill, without stopping. Two months before I could ride 10 miles even with lots of rest breaks. The 20 mile round trip to downtown and back home was an all day event because I had to stop often to rest. My legs and chest were always burning.
After a year I could average 12-14 mph nonstop for 10 miles. The next year, 15 mph nonstop for 20 miles. Then 16 mph.
Last year I had some setbacks: hit by a car, breaking my shoulder and re-injuring my old neck injury. During X-rays and diagnostics they discovered I had thyroid cancer. So I lost some fitness over the second half of 2018, and the first half of this year. Recovery was slow. I spent a lot of time in physical therapy.
I started doing interval training late last year. That hurts. There's no way around it. To make significant improvements in strength and cardio conditioning, we must push beyond our comfort zone. But it's essential to approach it cautiously and be sure your physical condition is good enough to handle it.
For the past couple of weeks I've occasionally averaged 18 mph on some familiar 20-40 mile routes, usually with one brief rest break midway. Usually I'm closer to 16 mph. Depends on how much rest I've had, and the weather -- I don't handle Texas summer heat as well as I used to, so my fastest times are at night or when I start at dawn.
It helps to have a goal. I'm not interested in racing again. But I enjoy group rides and I was tired of always lagging behind, slowing down the group, or just being dropped by faster groups that don't observe a no-drop policy. That was my incentive to push harder.
Mostly it makes the "slower" rides at 12-15 mph more enjoyable. I can carry on a normal conversation with other folks in casual group rides, instead of gasping for air.
And it's safer all around. I'm usually comfortable riding in traffic, since I started bike commuting in cities when I was 18. But it's not safe when we're exhausted, particularly riding at a walking pace on some streets. In general I've found it safer when I can average around 15 mph. This reduces the overtake speed with motor vehicles. We seem more predictable and drivers respond accordingly.
Best wishes. Keep at it and give it time.