Alex:
With the long time frame you are working with, you certainly can extend the 24-week plan. As for how much you can increase the volume of the endurance sessions, I’m afraid I can’t offer a blanket prescription. It depends on how well you’ve been handling the volume you’ve been doing. It is entirely likely that the plan, as written, is too light of a load for you. We consider the plan to be the minimum that we recommend a person be able to handle before tackling a big mountain objective. I’d suggest making small changes and seeing how you respond after a couple of weeks. A jump of 50% is too much for me to feel comfortable recommending to anyone.
With as much time as you have, I would recommend at least one 8-week block of max strength training followed by an 8-12 week block of ME. See if you do not see bigger gains by becoming stronger before doing the ME and hence get more out of it. That’s been our experience.
Scott