The upper Wye (above Rhyader) is a great stretch for training, however access to this section is now lost and is best avoided.
The main training section is from Builth Wells (car park is river right directly avove the bridge in the town) to Boughrood (park in the school car park river right immediately above the bridge). This section is 11 miles, and will normally take between 1 hour 15 mins and 1 hour 30 mins depending upon water levels.
The first 30 mins or so is fairly flat with some shingle rapids, good for working on driving the boat forward in swirly water - it is often very boily at high water levels (check the gauge on the Usk and Wye website - if it's off the top of the gauge it's good!). This is then followed by a number of very open wavy rapids, run river right. It's a great river for allowing youngsters to pick their own line as there is so much room, it is possible to completely miss the waves or charge straight down the middle if that's what you like.
Flatter sections follow until the main rapids section leading up to and through Hell Hole (run river right). You can tell you are there when you see the suspension bridge in the distance.
There now is access river left immediately above Hell Hole, possibly more suitable for younger paddlers who don't want to do the whole run.
For latest levels and access points - see Usk and Wye website.