I would only fair out of shield if the fox is in the air or at higher damage. It could be risky.
Zoning isn't just always hitting the opponent away from you, it is also about making your opponent scared of you/scared to approach you. You can zone best with marth by actually not attacking. Marth's attacks all require quite a bit of commitment. Even just jumping can leave you quite open to a fast opponent. What you want here is a nice dash dance game. Make it hard for him to tell where you are and don't give him an opportunity to get punishes in.
Don't jump forward when you are attacking. You pretty much always want to be neutral or moving away with your movement once you jump. Only jump forward when you start a combo.
An important part to marth's zoning game is leaving your options open (particularly your ability to move), and when your opponent moves in, you create a nice gap (most likely tipper range for a fair or something) through movement and punish them. To play aggressively, you can create this nice gap by moving towards your opponent and poke at them with late fairs, late nairs, and d-tilts. Keep in mind that you don't jump with the intention of doing a late fair, you just short hop nothing. Then if your opponent does something dumb, you attack them. Otherwise, you just do a late fair or nair so that it pressures them.