How good do you have to be to wear half tights

Have a nice butt.

javelin

Wear dark colors unless you have a large package.

An Intermediate Hobby Jogger

It's not about being good. You need to have the right physique to rock half tights (with no shorts over them): decent glutes, no thunder thighs, no chicken legs, no giant belly hanging over the front of the elastic, decent leg definition that can be seen through the fabric, and a nice package up front doesn't hurt.