2 years is a long time off and 1000 miles for a year is pretty low for a competitive type rider. I'm soon to be 66 and I have spent the last 9 months trying to regain some of my former abilities after retiring. I was never one of the fastest climbers but I could handle the rides and chase back on during flat sections. I want to be able to do those long hilly rides without feeling like quitting and I've had some success.
I can tell you at our age gains will be small and losses will happen. It takes a lot longer to build endurance and power than it did in my 50s. So much so that I even considered an e-bike or quitting the hard rides altogether. Persistence and patience are key.
I don't look at heart rate or any numbers, for that matter, and just rely on perceived exertion and how I feel after a ride. I do think if your heart rate doesn't come down after elevating it that's a sign your fitness is lacking. Your heart should slow back down quickly after a brief effort. If it doesn't slow down after a long, hard effort you may be dehydrated or just over doing it.