I launched straight into a 15-20% hill
yesterday without a warm-up and regretted it. It is 24 hours later and my heart rhythm has not yet fully recovered so I will not be doing that again. I have a gym bike and will do a 10-15 minute warm-up on that before tackling that hill again.
Even on flat rides, it takes 15-30 minutes before I start to feel okay (more quickly on warm days, more slowly on cold days).
I used to hit
this hill less than 5 minutes after setting off to ride back from work, so I always suffered riding up it.