Practice? Bah. Practicing is for sissies.
Actually, the other plus of 15-minute practice blocks is it's MUCH easier on your hands, and greatly reduces the risk of tendon damage.
Some stuff (drills involving building muscle response, not "learning" in the strictest sense) you need to taper into longer blocks - your alternate picking motion is what I specifically have in mind - but as a rule of thumb, go about your workout like a weight lifter - do sets.
-D