Inguilt is right but I also have some more to add.
Bananas, when in the air, are hit boxes that you can make linger, even if briefly. Try throwing nanas up while they approach and backing up with your other nana in hand. They will be forced to slow their approach and consider the banana that could easily fall onto them and cause them to take some damage from you. Take advantage of their hesitation and nana them. You could also Z drop nana's while you retreat to attempt to have them run into it.
Depending on the level of the marth, you could even just shield grab the fair but thats only if they aren't spacing well.
Here's an idea I had today after I read that you needed help conti.
Marths operate the best when they can stay at their maximum distance and hurt you while doing it. Obviously getting inside of their comfortable zone creates issues for Marth players because the speed he has without the range he has isn't TERRIBLY fast. What I mean is that marth is fast for someone who reaches that far, but you can still be faster once you've gotten past his wall. I thought about the way that marths use fairs, weaving and pressure and figured this might be worth a try. Lets use Link/Tinks invici-bombing technique, where you airdodge while z dropping the banana. When marth runs forward to close the gap and start pressuring you, wait until you can predict the fair, then jump towards them, and with good timing, MAYBE the air dodge will protect you while you drop the banana to trip them up and you can follow up afterwards.
I've never actually tested that last thing, I'm sure it could work in some situations but whether or not it could make a notable difference in the match up, I'm not sure.
Now that I think about it though, with diddy's bananas, the invicibomb technique might actually prove to be pretty useful at stopping a pressure fighter/character. Food for thought I suppose.