Is 5ft 1 inch short for a 12-13 year old?

The average height for a 13-year-old  is 62 inches , but it doesn't really matter because everyone grows diffrently and a diffrent paces . It could just be you will have a growth spurt :)

I think you might be a little tall for your age. Also depends on genetics and if you are a boy are girl. Most guys grow taller than girls.

Ibelieve this would be an obstacle.

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.