Most short cage rear mechs are only documented to work with a maximum of 27 teeth, so fitting that may work but you might find you have trouble with getting the indexing right. I'd suggest a 26 or 27 cog at the rear and a dose of HTFU, augmented with a bit more training to help build your strength. I've skinny legs after years of riding so you just need to get stronger to be able to get up these hills.
@andyp
Most short cage rear mechs are only documented to work with a maximum of 27 teeth, so fitting that may work but you might find you have trouble with getting the indexing right. I'd suggest a 26 or 27 cog at the rear and a dose of HTFU, augmented with a bit more training to help build your strength. I've skinny legs after years of riding so you just need to get stronger to be able to get up these hills.