First of all, good luck with your recovery from cancer. There are worse things than putting on 20lbs.

However, as you already know, losing the weight again will make things easier. And the advice given above about learning to draft is also good. Find a wheel and stick to it, riding as close as you can, and no more than a couple of feet behind. That makes a huge difference, and it is worth burning a match or two to stay there when the pace increases a bit, because once you're more than a bike length back, the energy required to get back on is much greater.

As far as training is concerned, it's not rocket science. Most of your time should be spent at endurance pace (Z2, since you know your LTHR.) Riding at tempo for an hour or so at a stretch is also effective at bumping up your threshold a bit. And, as you suggest, some interval sessions. To the best of my knowledge there's not really much difference in training strategies between us and the younger element, with the exception that we need more recovery time. I find that if I do a VO2 max interval session it may be two full days before I am fully recovered and able to go as hard again, and I'm nearly ten years younger than you. If you want to go faster you need to be fresh as well as fit, so I'd recommend not tiring yourself out with frequent hard sessions, but doing a couple of short but hard rides, well separated in the week, and going nice and easy the rest of the time.

And, as others have said, a 15mph average on a moderately lumpy ride isn't bad. There's always someone faster, but even as things stand you'll have plenty of people behind you.