I am a mt biker, too and have day dreamed about doing this for a few years now. never materialized, though.
You should read through the documentation here:
http://ardupilot.org/copter/index.html
There is a follow me mode:
http://ardupilot.org/copter/docs/ac2_followme.html
This flight mode requires a GCS (ground control station) with telemetry connection such as a 900MHz radio. There are GCS apps that run on smartphones such as Mission Maker and QGroundControl
…However…attempting to fly it on a densely wooded trail like that is setting yourself up for failure.
- your GPS accuracy will suffer from patchy precision and poor accuracy in trees like that.
- even worse - your GPS signal on your phone will suffer the same issues (now you have a flying machine that doesnt know exactly where it is trying to follow a moving target that also doesnt know exactly where it is)
- active object avoidance is being developed, but still largely immature. (Check out Thien’s progress on VIO & indoor, non GPS navigation here: Integration of ArduPilot and VIO tracking camera (Part 3) - Indoor autonomous flights and performance tests). However, it likely will not be sophisticated enough to fly fast through dense trees autonomously for a few years.
Your best bet would be to try a wide-open trail with minimal trees. Like Syncline Trail in White Salmon, WA for example.
Have you seen the Airdog and Hexo drones? they are developed from the same roots as Arducopter, but are designed specifically for this exact purpose.
Edit: I see you are a software engineer. Here is the git if your interested in digging around: https://github.com/ArduPilot/ardupilot
Edit2: Maybe try one of the Skyviper Journey GPS drones first. They’re cheap and built on the Arducopter platform. I am not personally familiar with them. I do not know if Follow Me is enabled out of the box, but I have seen several people update them with the latest firmware from the repositories and that would certainly allow the Follow Me flight mode