This is actually really simple. As we age, we detrain faster. You're seeing the normal numbers of an untrained person your age with your HR range. So what to do? Well, train. At first, it's only about the miles. Get your mileage up. Hills are fine. In fact, the dictum is "see hill, ride up it." Ignore your numbers on the hills, but watch your breathing instead. Keep it below panting on the hills. If you need to, get lower gears. We've all done that, Keep that HR down on the flat to recover. They say, increase weekly mileage by 10%/week, but I think 5% is better. One long ride/week where you ride away from home until you're tired, then ride back. As I just noted in another thread, your heart is the last thing to get into shape. Skeletal muscle is much more responsive. So that's the issue with your HR. Nothing's wrong, you're just out of shape. It'll come back, honest. Carry a small bottle of pickle juice with you for cramps. If you're doing it right, you will cramp, maybe only for the first few months. Shoot for 100 miles/week, say 50-60 of that on the weekend ride. Distance = strength.

You might want to do daily morning stretches to prevent injury, these stretches: IT Band pain (during ride)
You might also look up McKenzie exercises.