It depends on what you want to see- late August it will start getting colder in the higher elevations-we went mid August and there was already snow in some places in Denali NP. It looked like summer/fall/winter all in one day. As far as walking, it depends on the activities booked. I would book everything in advance so it is set and you don’t have to worry about it being booked up. Some excursions need to be booked well in advance due to popularity.

My payrents who are 64+, do they have to walk a lot for visiting places?
Do we ned to book the rides, like whale watching and other in advance or book on arrival?
Last two weeks of August is a good time to visit Alaska?

As this was a couples trip, I am not sure but as long as they can fly on the sea plane I would think so.