Haven't done one myself but have been to a few in my time, It will be a lot of work, but obviously they think yours is good enough to participate. Do you work a lot in the garden? If you could get some friends to help you out, particularly on the day, with teas and coffees and cake, but I find that most of us go for the gardens to get some ideas, I do anyway. If you think it will be too much then just decline, nothing worth it if you are going to stress yourself about it, just go to view other ones. Good luck.