Leg extensions will increase the size and strength of your quadriceps. Most gyms have at least one leg extension machine, so make use of it. While sitting you simply lift the weights by extending your legs.
For people over 40, it is important to stretch for at least 60 seconds before a workout. This is because the older a person is, the less flexible your muscles are. For people 40 and under, it is recommended that you stretch for 30 seconds prior to a workout.