The mileage is a factor because if you have to cram 10K feet into a shorter distance the climbs will have to be steeper. The hardest one I did had 12K feet in the first 75 miles. I don't know if it was what I ate or the altitude got to me but I suffered the last 15 miles of the climbing.

Also, the way the hills come is a big factor. I think a "front loaded" ride can be easier. I did a 10K century several times with a club supporting it and the climbs are steep throughout the ride. There is even a steep, nasty climb up to the finish.

Sometimes a ride sounds easier than it turns out to be. I did an 85 mile ride with about 8K feet. Deceptive, because there is 2500 feet in the first 12 miles, then lots of descending and false flats before the next hard climb. Just when you think the hard climbing is over there are several steep climbs yet to come. That ride is also susceptible to wind, so you can get hammered by the wind when you think you are on an easier section.

I think I have an easier time with lots of smaller climbs, 5 or 6 miles or less, compared to the first ride I mentioned which has a 35 mile climb.
In the summer we used to do an out and back on Angeles Crest which was 80 miles with 8K feet. There are 4 climbs of around 5 miles and 1000 feet each and a bunch of smaller climbs. Nothing too steep, like 5 to 6% for a lot of it. This is the easiest type of 100 feet per mile for me, even with 8000' elevation at the high point.