Hi Cindy, and welcome back!
It does seem so strange, doesn't it, to be writing bout all of our older cavaliers now as so many first join the board with young dogs. You are right, there is a plethora of information and archived discussion threads on MVD and heart issues and caring for dogs at all stages. As Kate says at the moment you have a fairly low grade murmur though I know grade isn't always directly associated with symptoms.
I have found MVD is so incredibly variable in each individual dog. While there is a general pattern for onset and for treatment, some really seem to stabilise for years while others will progress more quickly. Some advance gradually, some in fits and starts with long periods of stability.
I do really believe in having dogs fit and active to start with as then they are already ahead with a stronger heart than might otherwise be the case. I like to supplement with fish oils at 1000mg daily or so, plus have done CoQ10 at 30mg daily -- all before they have heart issues. I know there's evidence CoQ10 may not do anything, but the 30mg level is fairly low. I will say my Leo (who passed away in late Sept) surprised my cardiologist by remaining stable with a fairly advanced stage of MVD for three years. He was diagnosed around age 7 and held at a grade 3-4 for about three years. When he did finally decline it was rapidly over three months just as he turned 11, and given the choice of outcomes, I felt this was a blessing as only 9 months earlier he'd been happily doing hikes of many miles. He eventually was off all walks for the last three months of his life. But I tell his story just to show that despite a very enlarged heart, advanced murmur, etc some cavaliers can continue with normal levels of activity for a long time.
That said, I also know that dogs will not always indicate when they are really going beyond what they can do. I've seen so many dogs -- including Leo at the end -- continue to try and do what was their normal routine and of course most dogs will want to do whatever thei owner and other dogs are doing. I only realised Leo had taken a serious turn when we were on a long hike and we'd stopped to rest Tansy, who is smaller than the others and has a heavier coat (it was summer) and I realised Leo, who had been trotting along seemingly as normal, looked very tired. I carried him in a backpack the remainder of the way and he was so exhausted we thought we would lose him. It was a real eye opening lesson. I think even the most experienced owners can miss signs of exhaustion in some more stoic dogs (ad you'd be surprised sometimes at which are the stoic ones!) so the best approach I think is a mix of owner knowledge and observation, and regular consults with vet/cardio to know where a dog stands and what is appropriate.
I hope you have many long years together yet with all your dogs.