Go for it and have fun on your walk. If you are not exactly young in other words a bit older and only a recreational walker a bit of training would not go amiss.
We did a few walks over Mt Coo-tha (Brisbane) and back and we had no difficulty on the great ocean walk itself.
Chris Nash on the Great Ocean Walk
From Walk 91:
Thanks Chris, it was very nice to meet you and we are very happy you enjoyed your walk with us. The Great Ocean Walk is a fantastic holiday and we are really glad you enjoyed yourselves