Hi Kerby,
When I transitioned to a more CM approach with my older kids I did it on thing at a time. SCM has a great article about how to make a slow transition so its not too overwhelming. http://simplycharlottemason.com/how-to-get-started/transition/
As far as your specific questions…
Can I start w/ what I have?
Yes, but it might not work as well. Using the living books for example- you can use a dull text book and ask for a narration, but it is much harder to pay full attention and remember details to narrate when the material is not engaging. So, I would choose something easy and very living to start with narrations, for example Aesop’s Fables, even if you continue with the same old English text books for a while for the rest of the “subject”.
Can I work up to the timeframe needed?
Absolutely! In my opinion, I think most everyone needs to start at the beginning with these methods and move through them to the best of their ability, according to their maturity. The skills required to narrate, pay full attention, make connections and process information are much more difficult than simply regurgitating facts on a worksheet. Older kids often need time to develop these skills, it isn’t *just* because they are young that they need short lessons.
With older kids, I would start narrations with just one paragraph at a time until they feel comfortable with the short ones, them do two paragraphs, etc. They would probably be able to do longer passages relatively quickly.
If I had 45 minutes for each subject that would include everything, reading, writing, discussiong, thinking, narrating, etc.