Most leagues for 11-12 year olds establish weekly pitch counts around 80 pitches. I think it's important to establish and develop his pitch limitations, but not to constantly train at the limit. Due to substitution rules, it's not really beneficial to train an 11 year old to pitch more than 80 pitches. Substitution rules combined with league mandated pitch or inning counts often kick in by the 2nd through the 4th innings and 80 pitches is sufficient to cover that.
Be cautious when trying to "lengthen a stride." I try to have kids focus on gaining distance toward the plate. This gets them thinking more about getting away from the rubber with the back leg rather than reaching with the front leg. What often happens when they reach for the plate with the front side is a foot that lands on the heel and also lands open, thereby robbing the pitcher of stability and all the lower body energy created by the leg lift and stride.
This is another reason why I am not the biggest fan of the towel drill because I see that I have to retrain the stride mechanics of pitchers who have spent time on this drill and now reach with the front leg in order to hit the target with the towel. If anyone is using the towel drill or is thinking about adding it, just be mindful of bad motions that could possibly be engrained through its use. Monitor the drill closely when employed.
Sometimes I will simply get the pitcher to work on one-legged lateral jumps. I'll put a piece of tape on the floor on either side of their initial limits and take that measurement, and they will work to increase that range by trying to jump beyond the tape boundaries. It's not unusual to gain 6" of lateral leaping ability over a short 30-60 day cycle. That will automatically translate to the mound without ever having to work on stride length.