As someone who tried in vain to find a new casual summer dress yesterday (I only have one, which is ancient!), I can sympathise with TM on this issue. I'm slightly younger than her, though not much, and I couldn't find a single dress that I felt was right for me. It's partly an age thing, but mainly the fact that I don't feel that any of the casual styles around now suit my body shape (bits of it are thin/ skinny, other bits not).
My first thought when I saw 'that dress' were similar to the OP's, but then it crossed my mind that she probably has similar problems to me in finding something perfect. It was casual, looked comfortable, and was probably perfectly suited to the activities she is planning to do, so go for it TM! Wear whatever you want!