OH RIGHT MARTH.
Ahem, my friend mains him and I used to be able to beat him when he used Marth, but no longer. Marth is one of the worst match-ups for Ganondorf, only being topped by Sheik, Falco, Meta Knight and DDD if he uses his semi-infinite.
Basically the only thing that we have that can outdo Marth is the dtilt, but a Marth most likely won't be going for a grounded approach. If a Marth walks toward you, you can expect any form of ground attack, and if you can space the dtilt well... You should be able to outrange him.
Fighting against Marth needs extreme patience... A good Marth can space themselves so that they can stay out of your range.. Ken Combo can be used for this. You can hit him with dtilt when he lands (if you powershield both hits), but it's not super likely.
Thunderstorming Marth can be hard as well, since Marth won't be sticking with grounded approaches. If he uses a dash attack, he's all yours, but don't count on it. If you can block any of his smashes, you can get to him. However, I don't think you can do much good from it. A downsmash can also be thunderstormed, but same with dash attack, don't count on it. Neither attacks will be used often by Marth.
Flame Choke can put a number on Marth. If you can get the first one in, you can usually get multiple: Marth's rolls don't go that far. His get-up attack is also very low and thunderstorm bait. Although, there are no guaranteed follow-ups, so watch out for that.
Another okay tip would be to get Marth directly above you. The only attack that a Marth can do against your upair is dair or a counter. Neither will be used often. My training partner always counters a lot since I always blindly attack him. >_> A clever Marth will airdodge, which you can read and attack him out of.
Use Marth's light weight to KO him early. I'm not sure if Flame Choke -> iDA works (it doesn't), but a cleverly placed Dash Attack can kill him and sometimes out prioritize him... Don't take me up on this, since I'm not 100% sure. Marth CAN be gimped.... But I wouldn't try too hard. He can stage-spike you with his up-b and it's really fast. You can try to spike him out of it, but it'll either be luck or extreme expert timing.
Ken Combo shuts down a lot of your game, but with patience you can get through it.
I personally rather use Wolf against Marth, which is a very tight and fun match-up. Any 50:50 match-up that isn't a ditto is okay in my book.
As for a match-up number... 80:20 for Marth sounds about right.