If you are getting zoned Pit does have options, but they are all risky. Some are risky as hell and will get you murdered. Others are only slightly risky.
One of the best options is SH back and do an arrow. This allows you to reset your spacing and put pressure on your opponent.
There is also doing a safe roll. Safe roll is when you buffer roll after an opponent hits your shield. If Marth does a rising SH Fair and hits your shield you need to roll ASAP since yu can get away safely.
BUT...if Marth did a SHFF fair or a d-tilt you need to think fast and act cautiously. Those moves are frame-traps. This means that Marth recovers quickly from the attacks and he can react to pretty much anything you try. Against a higher level Marth, if you roll towards him you will be punished. If you roll away you MIGHT get away, but even if you did more then likely Marth will close the gap and reset the zoning game.
Most people who play Smash are not at the level that they can abuse a frame-trap. But if you ever hit the upper echelon of gameplay be prepared.
Anyway, the best thing to do is just mix it up in that scenario. Sometimes roll, sometimes spotdodge and sometimes, SH away. You are at disadvantage, but it's better then sitting in your shield and getting your shield poked through or getting grabbed.
You could also get risky and go for a dash grab/attack. This can be made more effective if you use instant dash attack, which is C-stick down as soon as you run. This is can be pretty surprising to an opponent. It's unsafe on block though, so be cautious.
Also correct if I'm wrong but doesn't Pit enter his full run animation almost instantly? If so he could dash to shield and block attack that an opponent would use to keep him pinned down. Then he could net a shield grab and shift the momentum of the fight.
I'll mess around with Pit some more and see what else I can come up with. Please experiment with my methods to test their effectiveness.
