Do you have to stay in Anchorage all the time? If you didn't mind a couple of nights in Seward, you could go there after the Whittier tour (you've already driven 40% of the distance) then have your "down" day in Seward.
I'd also suggest that you could save several hours of (frankly not very inspiring) driving by simply taking a Denali flightseeing trip from Anchorage instead of Talkeetna. It will be a little more expensive than the Talkeetna flights, but you could conceivably save a day's car rental. Look at
Rust's out of Lake Hood - a veteran operator with a good selection of options. In addition to the mountain, you'll see a lot of wilderness country from the air - it's very impressive.