You can substitute a ride with a brisk walk. I found that at least for me, walking (sometimes speed walking) has given me good climbing endurance. I walk between 2.1 and 2.8 miles in a half hour, twice a day on my non cycling days. My dog loves the walks and they do help me with conditioning.
For a few months, I stopped the walking, thinking I didn't need it and just rode more. Not only did my endurance suffer, my dog was unhappy..... almost depressed. It's all good now, try it.