You do not need to move quickly. After a week or two of conditioning you will find a comfortable pace. Then, increase the time you spend in the activity according to your performance. Do this every day. Increase moving time about five minutes a week after the first two-four weeks.
Weight training is very helpful.
However, there is some disagreement on what age a child can start. One theory says that a build up of muscle mass while bones are growing can limit growth. Some tall body builders disagree, and use themselves as an example to the contrary. Make sure you are well supervised and get advice if you weight train. Any physical education teacher and most coaches can show you how to do push ups, sit-ups, and other various strengthening exercises.