DId you do any damage.. Well, that is a complex question but as you asked, EXACTLY as you ask, yes you did.. But read on.
Did you do SERIOUS damage.. No, you did not EPICALLY if they are true DEEP CYCLE (Like GC-2) Batteries,,
Explanation: Battery life is rated in charge/discharge cycles. So at the very least you did "one cycle" if you take it down too far,, And how far that is depends on the type of battery.. Starting - Marine/Deep cycle - - many - - true deep cycle. (That's many dashes) determines how far you can take it down before you do SERIOUS damage..
Charge them, at least 8 hours with a smart charger, and see how they stand up
On my GC-2's I've run them down to teh OH S**! Level more than once over their 8 1/2 year life (Starting in the first year) and .. Well they lasted 8 1/2 years.. For the make of batteries average life is 7-10 years. (7+10)/2 is 8 1/2.. In short, they fell dead center in their expected life span range.
I'm happy.