Don't be downhearted.
My girl was 8 when she was diagnosed with CDRM. I put her in a cart when she could no longer walk with her back end and I had her a further 18 months. I chose to put her in the cart because she was so young and had no other problems (like arthritis).
IF your dog does turn out to have CDRM the timescale before going completely unable to walk/support themselves is very variable. I have known dogs go off their legs within a couple of weeks (this is rare) and some go a couple of years or more.
I always said with my girl that although I knew she had CDRM at least we had some good time left together. I previously lost her sister at 6 years old with bone cancer in her hind leg, I only had a four days from diagnosis to having her put to sleep and she was in terrible pain in her last day. I reasoned that with this girl who had CDRM then she could well get something else that would cause me to loose her before the CDRM took hold.
Sadly when dogs get to this sort of age we all know that we are going to be faced with making that terrible decision in the very near future. My boy is now 13 (breed average lifespan is 11) and I know each day is a bonus, it is hard and I find myself watching him like a hawk for any limp, sickness etc.
Don't take this the wrong way I mean it in a nice way. When you say your dog may have this awful disease that has no cure - in a way they all have a terrible 'disease' with no cure - old age. Sadly it is something we all have to live with, we will eventually loose them one way or another.
CDRM is not painful for the dog and in that way it is a kind disease.
Try not to be too upset/down around her - dogs often pick up on our moods and act accordingly.
Swimming is a good exercise for CDRM - do you have a dog swimming pool anywhere nearby?
You might find this thread interesting (Jake has CDRM):
Jake's hydrotherapy
Sue