1. Voy a hacerlo: the meaning is "I am going to do it", I plan to do it some time in the near future 2. Lo voy haciendo: means "I am doing it", I am already doing it, and continue to do it for some more time

1. Voy a hacerlo. I'm going to do it in the near future but i'm not doing now 2. Lo voy haciendo. I'm doing it in this moment. You could say "lo estoy haciendo" but the meaning is not exactly the same. Actualy "lo voy haciendo" means that you are going to do it so nearly that you are doing now or you are begining some action to do it. For example: I'm watching TV and I say "Voy a comprar el pan", then I turn off the TV and stand up. "Voy comprando el pan" is not correct. I'm watching TV and I turn off and stand up, in this case I could say "voy comprando el pan" because I'm not buying it now but I'm doing an action to buy it. I also could say "voy a comprar el pan". Usually we use "lo voy haciendo" when there are some things to do and one of us does one thing and another one does another thing. For example Mientras (tú) traes el equipaje (yo) voy sacando el coche. Another example -Voy a darme una ducha y luego vemos la película. - Vale, yo voy sacando el perro (mientras tú te duchas)

1. Futuro. Voy a (going to) 2. Se realiza en el presente y se continua haciendo en el futuro. Tambien puedes decir: "Lo estoy haciendo".