We loved this trail. We took it from Turtle Bay up to the top of Shasta Dam on a cool Saturday morning. Round trip was a little shy of 40 miles, with a fair bit of climbing. (Heart Rate Hill is aptly named)
The trail is immaculate - smooth and clean, busy in sections but mostly not too bad. Big stretches of it take you right along the river, and there are some beautiful spots. We saw tons of wildlife, including otters and a bald eagle. The views from the dam were awesome as well. You can do the whole trail, as we did, or you can break it up into sections - there are a ton of easy access points.
Overall, I can't recommend this trail enough - it's a gem. The only caveat might be to start early in the summer - the heat can be miserable in Redding. And if you do end up making it back to Turtle Bay after it's gotten hot, you can get a nice cool Coconut or Mango ice at the entrance to the Arboretum when you get back.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.