The horse that I was training for western pleasure was naturally long strided and had a 'motor'. I can't agree more with gizpeptig. It turns into a habit of going slow. Horses learn through a good release/reward. You need to make it clear what you want. Before anything, your horse needs to be collected. If your horse is going too fast, simply back your horse up while saying "whoa" while you back, turn him around a few times and ask your horse to step off again. You can start this on contact, and as your horse starts to understand, give him some more slack. ALLOW YOUR HORSE TO MAKE MISTAKES; DON'T MICROMANAGE. Let him take 2 or 3 strides too fast so that he knows why he is being corrected.
Also, if possible, ride more than once a day. End your rides on a good note with plenty of reward, they are more likely to remember what they were taught the previous day. But remember, just because they do it good one day, doesn't mean it's been taught. Consistancy is key.
Hope this helps.