I wouldn't judge yourself yet, just 10 days out from radiation your body is certainly not performing anywhere near its potential. And sounds like you are 6' 1" and have gotten up to 220 lb, that isn't too bad compared to the alternative - most chemo/rad patients lose a lot of weight and not in a good way. I will bet that after a month of riding, even without making any special effort to be faster or lighter, you will be (faster and lighter).
Improving your aerodynamics will probably give you the biggest bang for the buck. Ride in the drops, bend your elbows and/or lower stem to get your back as flat as possible. Stretching and yoga may be required if you've been getting less flexible with age, as we do. Avoid flappy, blousy clothing.
Possibly your gearing could use some tinkering. There is likely a particular cadence where you are most efficient, the cogs at the little end of the cluster need to be close enough to let you match that cadence to the speed you are trying to hold.
Losing weight will naturally help a lot on even a slight grade, but has much less impact on the flats.
As far as training goes, in addition to long rides to build endurance, I would do shorter rides where you hold as high a speed as you can for a set distance (like a mile), recover, then repeat, making the speed higher and the distance longer on each ride. Normally on the road there are periods when you can rest a little - road slopes down, wind eases, you draft someone - and periods when you have to hammer. The hammering periods are when you keep, or lose, contact. So you basically need to build up your hammer, make it longer and bigger.
Last summer, I was able to do average-while-moving of 17 mph for long rides, longest being the first day of Seattle to Portland (98 miles at avg 17.03 mph). No wheelsucking for most of it :-( That basically means cruising at 20+ mph on the flats to make up for the slower parts. At 20+ mph, aero drag is the biggest enemy. I spent almost 4 hours of the 5h48m ride down in the drops. If I'd been on the hoods - no possible way.
Use this calculator to see the power required to maintain different speeds in different riding positions.
http://bikecalculator.com/