I once read a tai chi article by a guy who said if you practice the slow Yang form for 20 years, you'll be able to fight when a self defense situation arises.
No you won't. You'll be seriously injured or killed. That's what will happen.
If your practice is only on form, you're missing the heart of any internal art, which is self-defense. You must practice fighting applications in a way that helps you internalize them so that you can react without thinking.
Sparring is one way to do it, but let's face it. Sparring the tournament way limits your techniques. And you can't really use all the techniques of these arts against classmates because you can't drive your palm through their faces, break their elbows and wrists, dislocate their shoulders, twist their necks until they snap, throw them to the ground to smash their head, etc.
However, you can practice the techniques to a point. In order to pull off a fighting application, it often takes the right entry, the right angle, and the right reaction depending upon how the opponent is stepping toward you and what they're throwing.
In my classes I sometimes like to put students into the "Circle of Death." One student gets in the middle of the circle, and I call out an attack and a defense. For example, if Chris is in the center, I'd say, "Chris, defend with Pi Chuan. Kim, attack with a lunge punch." Kim immediately attacks and Chris has to defend.
The defense has to be done with good body mechanics. The strike or take-down is softened but if the body mechanics are correct, the power will be there without driving it through the opponent.
Next, I might say, "Chris, defend with Heng Chuan. Kim, attack with a front kick."
As students go through this exercise, it helps them internalize how to use the applications. They have only a second or two to think about it. Many beginning students get a little flustered, and they don't always use proper posture or mechanics in their application, but over time they get better, especially if the teacher stops the action and shows them the mistakes their making. Perhaps they're losing peng. Perhaps they're using too much arm and shoulder muscle and losing the whole body connection. Perhaps they're not striking a good target.
You can do this drill even if you're practicing with one partner and a teacher isn't around. Have the partner call out the attack and what you should defend with. Coach each other as well as you can on proper mechanics, posture and technique.
When you are practicing alone, you can still go through this drill. Imagine an attacker, and think of different attacks with empty hands, kicks, and weapons, and try to react in your mind as your body goes through the motion. Even sitting in a chair somewhere without going through it physically, you can practice with your mind and take yourself through the defense using the proper body mechanics and power. Mental practice like this is almost as good as practice with a partner.
Now, for those silly people who say that practicing with partners like this without hurting them doesn't work---that's what heavybags and other devices are for. I work my power on the bag and other devices (the punching bag "Bob" is good for visualizing targets, for example).
I'll have a video lesson soon on how to deliver power in a fighting application with a partner while avoiding injury.