You could change the rear cluster?
That is what I am doing. Tomorrow the 12-25 rear cluster is being replaced with a 12-27 (the chain is new, so shoudn't be an issue). I can ride any hill with the 12-25, but on a longer ride like this I want some "some lower gears in reserve". I used a gear chart to determine that this will give me a gear ratio similar to the lowest gear I used to use on my hybrid bike on these types of rides.
My decision was also based on reading the post on this topic:
Only a month (+ a bit for me doing the 5days option) to go