It depends a bit on how fast you are biking. The downhills can be brutal if you are on a bike, with the extra pounding.
But if you are doing technical trails, and aren't going super fast, a dog like yours could do 20 miles with a bit of conditioning. Overheating is always something to watch out for. If she's off leash, she can self-regulate and jump in creeks to cool off and get a drink.
I go early morning to avoid traffic and for the cooler temperatures. Always watch the dog, and gage distance and speed by how she's doing. I'd stick to dirt trails, but if you're mountain biking, I assume that's a given!
And do teach her trail manners. Bikers can be vicious about sharing their trails, don't give them an excuse to complain about your dog, yield whenever possible and keep the dog out of the way of others.