I'm afraid you can't do a lot about it now. The only thing that might work, is to put it in the microwave for a couple of seconds, right before you eat some. Perhaps putting some syrup on top could help too. But I guess this would be more helpful with the mouthfeel than with actually softening your 'brick'.
As for next time, if you did like the taste and want to use the same recipe, use a smaller baking tin. Also check for doneness 5 min before it should be done (by inserting a toothpick in the middle). That way you can have an estimate how much longer it should be in the oven. Or as Jay suggested, look for a recipe that tells you the size of the baking tin.