The change in distance from flat is fairly small.
A 10% slope is about when you start toe striking instead of heel striking. Your leg is bent when your foot touches the ground and you have to straighten it. This is a lot more work. Your distance is about 0.5% extra due to the slope.
A standard stair has a rise over run of 7 over 10. Now you are going about 12% extra distance.
Typical scree slopes are at the angle of repose -- about 35-40 degrees. Now you are going about 25% extra distance.
This however is a bad way to look at it. Going up and down takes a lot more energy than traveling horizontally. When you are walking on slopes of 8% or lower, the biomechanics of your leg swing, and your stride length determine your speed. With some training you can lengthen your stride and increase your pace.
Around 10% slope you have to use a different motion, that is less efficient, much higher energy. For most of us at 20% slope (steep hill) conversation is short phrases separated by puffs. (I knew one lad who could sing going up a 15% slope. He also played the bag pipe. Another could carry on a conversation at slopes and speeds I could do little more than grunt. He had planted trees on mountainsides for 5 months.)
Coming down isn't a lot better. Your muscles are not designed for absorbing energy. Coming down hill you often need to take time to pick your way. A long descent can leave you at the bottom with "jello legs" quivering from the effort of contracting under load. This too you can train for, but a stair climber machine doesn't work. You have to go down slopes or stairs.
For running trips in one sense I don't care how far I really traveled. I want to have a good idea of what time it will take. So I have an estimater for the 'effective distance of elevation change'
In terms of planning distances I figure 5:1 for elevation. That is, a meter up effectively adds 5 meters horizontally. This is true for up and down both.
In practice the up part takes longer for any but the most fit, but coming down is still slower than flat (you are picking your foot landing more carefully.) Both going up and going down you are taking shorter steps.
So a 20 km loop route with a 1 km total of up and down will take about the same time, and leave the same fatigue more or less as a 25 km flat route.
In more detail: 8% is a magic number in terms of gradients. Up to about an 8% grade (8 foot climb per 100 foot horizontal) you can walk normally. You have a heel strike. Your step shortens up hill and lengthens down hill. At 10% for most people, you change to a toe strike, and you are climbing, rather than walking. This is not nearly as efficient. You will find that the vertical change entirely determines your speed. E.g. when you are walking you do 3 km/hour. When your are climbing you do 1200 meters an hour up. Your numbers will vary.
But the 5:1 rule is good enough for me.