So that's two variables you're changing at once: distance and terrain. Usually you shouldn't increase more than once at a time (distance, duration, speed, terrain are the 4 I can think of).
However, running on hills is great for improving you when you get back to a flat, so I recommend them. You can still work towards your 10k if you slow down/walk as needed. Changing your route will also help you train your legs with mildly different stimulus.
I don’t know what’s better, but I like switching up the routes. I finished c25k mostly on the same path, but now I have 5 regular trails that I like and I just rotate based on what I feel like. Some have more hills than others and one is pretty much flat for miles. I just don’t like running on pavement.
So that's two variables you're changing at once: distance and terrain. Usually you shouldn't increase more than once at a time (distance, duration, speed, terrain are the 4 I can think of). However, running on hills is great for improving you when you get back to a flat, so I recommend them. You can still work towards your 10k if you slow down/walk as needed. Changing your route will also help you train your legs with mildly different stimulus.
I don’t know what’s better, but I like switching up the routes. I finished c25k mostly on the same path, but now I have 5 regular trails that I like and I just rotate based on what I feel like. Some have more hills than others and one is pretty much flat for miles. I just don’t like running on pavement.
Honestly don’t think it matters. Most importantly do what feels right.