Welcome to the HTF. I'm sorry you & Lucy are going through this difficult time.
Incontinence is normal and can be dealt with. Vision & hearing problems can also be dealt with by dogs who can continue to lead a good life.
You want to make sure Lucy is not in any difficult pain. This is where your vet comes in. I read where you write that she sometimes has difficulty with steps, etc. But that can also be normal. My 52-year-old knees aren't what they used to on certain days either! Your vet should be able to guide you with how Lucy is feeling day-to-day.
And it's not unusual for older lethargic dogs to sort of forget who they are for a moment and start running like a puppy. It sure is nice to see those signs of life at certain times, right?
My wife and I have one rule of thumb which goes to whether the dog still lifts its head with signs of acknowledgment when you walk into the room.
Dogs are quite stoic when it comes to pain. Some of them really don't let on when they are hurting. If only they could/would tell us! That's why it's important to bring your vet into the loop as you start to ponder this decision.
One thing that is for certain is that it is up to us as the dog's owner to be certain to do what's right. Unfortunately, the dog cannot do this on his/her own. It is an important obligation we have as pet owners.
You and your family and Lucy will be included in prayers.