I can't quote you an exact mileage...but it's not far. I would guesstimate it at 2-3 minutes tops at a slow-to-moderate pace. I definitely think it is less than a quarter of a mile.
Also, once you get off the pier at the boat drop off, there are benches scattered around if she needed a rest. I only mention this because I think the pier has a
slight incline as you go towards land...which could potentially be a concern.
Edit to add: Google says it is a tenth of a mile.