As a Garen main I find that matchup very challenging. The goal is to either snowball with the help of ganks, or survive until mid game where the goal changes to being more useful to your team than Darius is to his team. Even in that latter case Darius usually pulls ahead in lane, but falls behind afterward far enough that I can safely fight him one on one.
The big problem is the raw power Darius has early on. Later in the game, with both being equal, it's actually a close fight, but Darius has that early advantage and can easily snowball on his own.
Just end the lane phase as soon as possible, get your allies into a better position by roaming and giving them kills (you're Garen, all you need out of the ganks is a little experience and no deaths), and you should be good to go. If your team does well Darius will be forced out of top lane by his own team needing him. If your team doesn't pressure objectives he may very well stay up there for long periods of time throughout the mid game when he still has the 1v1 advantage on you.
Make sure to time these roaming ganks well so you don't miss out on too much farm from your own lane, otherwise Darius will become a menace you will never be able to fight. If these ganks fail your allies will not be in a better spot and you'll be forced to return top where you are at a disadvantage and can be easily zoned. Remember that the reason Garen does anything in team fights is because his damage makes him a threat to the enemy team. If you fall behind you are not a threat, and you will do literally nothing besides secure kills with your ult.
TLDR: Assuming equal skill and knowledge Darius has the innate advantage, but Garen can scale harder. Who is winning only flip flops, there is never a middle ground at any stage of the game where they are equal.
EDIT: I've not really felt the same thing with Riven though. It does feel fairly one sided for a while, but if you play safe early the middle stage of the game always feels incredibly even, with fights being decided based on who makes a mistake rather than who has hit what power spike. That's just my experience of course.