It so depends on the trail. I don't think avg mph is important unless you are racing and comparing yourself to your peers on the same course.

My speeds are all over the place - 2 mph if it's really tight and technical and over 30 on fireroad descents. I love the tight, narrow singletrack of central Oregon where you may have to do a split second track stand to make a right angle turn between boulders or to get the right angle your bars between trees.

It's both fitness and skills that determine your speed on a trail. You have to have both to ride faster. Descents are all about skills and mind set, not fitness, so get the trainer for fitness but you have to ride frequently to get the skills.