I agree with your concerns. Also they have to cast spectacular actors because the actors in this were phenomenal.
One way they could do it though is show the old woman in the retirement home, as she gets flash backs and sometimes gets her memories back. We gradually get to see Noah as an old man get intorduced. That way they could play around with time. As it might only be a couple of hours in the home, where she could have thought about a lot of memories in that time. Obviously if the series gets renewed and gets it's final ending, it would have to finish where the film did with their deaths.
I agree, the casting can be tricky. Let's just hope they get some serious talent.
As for the story, they're gonna stay away from the elderly version of Allie and Noah, as well as the present day I believe. The CEO of CW, the network behind it, said: "I do not believe we’re going to see the older couple."
Guess we'll just have to wait, see and hope for the best.
The older couple were great. Nothing against old people. My parents are old, but the main reason I liked the book better is because the characters were only old at the beginning and end of the book. The movie kept going back and forth.