If only young/fit people use the infrastructure then the infrastructure is really poor! Note than I'm from the Netherlands and comparing countries is a bit weird. Why not aim for a better infrastructure than the Netherlands?

Anyway, for hilly areas I expect a large share of people to use electric bikes. And way more care for safety. In Netherlands they think about possible safety issues. Meaning, someone on a bicycle going down a huge bridge is probably going fast and doesn't want to stop. You could argue that a bicyclist shouldn't do that, but that's pretty pointless. What's better to engineer the infrastructure in a way that deals with that. Hillier areas would require a lot of thought.