Why get married? Do you want to start a family? Or make things easier on your kids? Marriage won't fix a thing, just makes it more complicated. Seems to me, as a married woman, that you should have a reason to walk down the aisle-- especially if you don't just want to do it to be romantic. Marriage is wonderful, but you have to want to be married to make it. If you don't have a reason, keep dating!

I, too, wonder where the desire to marry is coming from in your situation. On the practical side (aka for health insurance and tax benefits), being legally wed carries benefits. On the flip side, there are people who think exchanging vows to be unnecessary signs of their commitment to one another. Maybe this is your situation, in which case, I say keep dating.

Reasons you might want to get married:
to promise to stay together forever - which may also give you the security to do things like move together to make your commitment to each other public to get the support of your community for your relationship to have a fun celebration (your way!) financial and legal advantages for kids to make public vows that you find meaningful (not necc. forever ones)

I think if you've been together for 20 years then you have a pretty solid relationship. You don't have to get married to prove you are committed. But if you really want that wedding and the legal benefits that come along with it, then go for it.