So coming from a mountain bike to a cx bike which im riding on the road....
Today for the first time i did a fair bit of climbing and i just couldnt spin the lowest gear ratio on my bike, really struggling to get up the hills and it hurt my knees mashing the hard gear, i had to get out of the saddle a lot, just dont have enough fitness to climb.
Try going back to the mtb on the road and train yourself out of the bottom front ring, with y hybrid ( 48, 36, 26 and 11-32 in 9 cogs), to start with i had to use the middle front rings for the local hills and dips (nothing too severe) and it took quite a bit of effort to every week only use a smaller rear cog each week, until i was comfortable to change up to the big ring at he front (and to a large rear cog) and then start the process again.