It's a big question that depends on so many different aspects several of which are hard to predict. It's hard enough even for people who don't have a progressive neurological condition in the mix.
My situation has some common points with Nocksy. I'm 48 and have worked 27ish years for one employer. (several different jobs in that time). Since dx I have had lots of opportunities to apply for voluntary redundancy. I haven't pursued it because I feel like I still have something to contribute.
Its reasonable to adapt your work to fit your condition. This could mean something minor like flexing hours or location of work or something more fundamental like moving to a different role in the same organisation.
What aspect of the job are you struggling with? Is it a physical symptom? or fatigue or depression? can you adapt around them?
I have worked for 9 years since dx including job moves within the same large company. I too would love to hear of others experiences especially about the effectiveness of the equality legislation.