Well, a lot of the answer may depend on which country you're talking about.
Assuming we're talking about the United States, the first question would be whether or not the uses of the mark present a substantial risk of confusion because the marks are confusingly similar and used on similar classes of products. If there's no risk of confusion (like if the mark is "Ambrosia", and one use is for food while the other is for carpets) then it doesn't matter whose right to use of the mark has priority, since there's no infringement.
Additionally, in the United States, if the products are sold only in one state or in a limited area encompassing a few states, and the mark hasn't been registered, then the users may both have the right to continue their respective use of the mark within their own territory, but not have the right to expand further into each others' territory. As a famous example, there's a Burger King in Mattoon Illinois which is not at all affiliated with the much more famous Burger King brand and which is allowed to continue operating due to the fact that it opened in the period between Burger King going into business and the registration of the Burger King mark, and entirely outside the market area that Burger King had at the time. http://en.wikipedia.org/wiki/Burger_King_%28Mattoon,_Illinois%29
There's also the possibility that the mark is simply descriptive or else generic, in which case it may qualify for no protection at all.
But if the uses are confusingly similar, and the products are being marketed in the same territories, then the priority of the usage becomes important, since the usual rule is that a prior user will usually win. The question here would be whether, in that 20 year period, you might have abandoned the mark at some point, since if you had ceased to use it for awhile, your priority would count from the most recent continuous use (most likely). You've also used it on several different products. If the products are in the same general category, you could have an argument that this was a continuous use of the mark in that class (for example, Coca-Cola has used the Coke trademark on several different versions of its famous cola, some of which they've stopped making or reformulated, but they're all cola soft drinks, so they can be counted as using the mark for a very long time.) On the other hand, if one of your products was a knife and the most recent one is a tiki statue, I tend to think the court wouldn't be inclined to agree that your use should be counted from the time when you were putting your mark on knives.
And of course, the answer may change completely depending on what country we'd be talking about here.