The Idea: A mobile app that allows you to track your location and alert friends/family if you don't return after a set time.
Deep Dive: The key to this app is to make it as simple as possible. You would be able to give users the ability to start/stop a hike and put in their expected time of return. You would also have an interface for users to enter in their emergency contact information.
The issue here comes with cell phone reception. Given that most hiking spots are in remote locations, you would have to store the user's location at a set interval and send an alert if it hasn't significantly changed after a set interval of distance and time.
Validation: This app is in the similar space to those "life alert" type devices. You could start simple by only providing a start/stop button with an expected return time. If someone doesn't return to their location after a standard deviation from their expected time then send out an alert.
Monetization: You could have targeted ads for camping gear and running/hiking shoes.
Backstory: In an effort to get back into shape, I've been trail running or hiking at least once a week. On my last outing, I slipped on a rock and injured myself enough to only be able to walk the rest of the course. Thankfully I was almost finished but I realized I hadn't seen anyone else in the past 30 minutes and nobody knew exactly where I was located.