Not Bad. The Marth you were playing wasn't very good, however. Horrible gimping lol.
A few things off the top of my head:
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-You can't d-tilt spike Marth.
-Dash attack is very undependable as an approach. It's relatively slow and you can react to it easily. With Marth, for instance, you have enought time to see it and counter it, shield grab it, and even Fsmash it. You should only use it to punish significant lag when you aren't close enough to do anything else.
-Look a little bit further into Pit's ledge game. Hanging on the ledge, Fair isn't great since you expose your head to Marth's sword. Uair is a better option to damage people who are too close. If they wait for you from far away, you can release, jump, arrow.
-Try following up your attacks with aerials more often than with arrows. At high percentages arrows are good since you can't chase them after a big hit, but from 0~90 you should be chasing your opponent after hits and keeping them up in the air.
-Pit's grab range is shorter than you may think.
-Don't use arrows when the enemy is nearby. At medium range Pit should wait for an opening and attack with quick attacks.
You are doing great for having used him one week. Keep up the good work and arrow gimping :D and post more videos!