You'll want to check if you need permits before touching it - some areas have wetland regulations around ditches with cattails, and that's where a contractor actually earns their fee by knowing what's allowed. That said, renting an excavator and doing it yourself isn't impossible if the ground's accessible and you've got decent equipment skills, but 200 feet is genuinely a lot of work. You're looking at multiple days minimum, plus disposing of all that silt and vegetation.
The timing matters more than most people think. Late fall or early spring when the water table drops is ideal - you want the ditch dry enough to work in but not so late in summer that you're digging through hardpacked mud. Winter's brutal if you're in a cold climate because frost makes everything worse. If you do hire someone, get it done in fall so spring runoff flows properly.
Here's something nobody seems to mention: before you rent anything, rent a small pump first and actually clear out the standing water so you can see what you're dealing with. Might cost you 100 bucks for a day, but then you know if it's just sediment buildup or if there's something blocking the outlet (collapsed pipe section, debris jam, whatever). If it's just silt, you could potentially get away with a smaller excavator or even a backhoe instead of a full-size machine, which cuts your rental cost significantly. Sometimes the problem isn't as big once you actually look at it.