Done a spot of track building myself in my day. It was mostly cutting the trail though, so loads of block and tackle pulling out tree stumps. That's as far as I got before "f**k this shit".
This is key in my eyes to drum up interest.
Going to try and get 3 boys from here down. No idea what level they are at. Probably 30 odd year old green route riders.
I advise using an aftermarket wrap round case instead of the water resistant bulky jobby they come with to make the sound better.
I have yet to try a mic (I plan to get one soon) but the lack of clear case helped a considerable amount.
https://www.amazon.co.uk/Camera-Standard-Border-Protective-Housing/dp/B00HE7Q9U2/ref=sr_1_10?ie=UTF8&qid=1471962564&sr=8-10&keywords=gopro+frame+mount