Hi desi. We did the 3+ mile walking tour in Florence a couple of months ago through an excursion offered by another cruise line. Yes...depending on where you are from (we're from New England), it may be a difficult walk in August. If you just want to see David, one option is to take the ship's transportation-only excursion to Florence or go by train, and visit Galleria dell'Accademia on your own. You can then see other parts of town at your own pace.
For more on train travel in Italy, should you opt to do so, we have a great thread located here
By the way, we were on Century in northern Europe a few years ago, shortly after her refurbishment, and loved the ship. Each year when we plan the next summer's cruise, the first thing we do is see where she'll be cruising.