I was told that the key to trail running is: keeping about the same
amount of exertion/effort throughout (whether uphill, downhill or
flat). Is this true and is the best way to judge this the breath/
breathing rate?

Also, what do people think of a rucsac type hydration system (eg.
Camelbak) versus a hip mounted bottle holder (eg. Lafuma). In other
words, weight ditributed across shoulders, back and hips (but sweaty)
versus a lot of jangely weight on the hips? What's best? Opinions,
please?

Cheers. Vince