I'm going to break this answer into three parts. One for each type of travel as I see them. There are likely more, and you should see this as a spectrum rather than distinct categories, but I hope this gives you a good foundation.
Travel purely to get to the destination
Sometimes the party is traveling purely because they were at location A and need to be at location B for the plot to continue. Location A and B are X days apart because you want your world to feel large and realistic. However the party are strong enough and/or the the region is safe enough that this journey poses no significant threat or point of interest.
In these cases I encourage you to do what you have already been doing:
sometimes I just say it's been 3 days since you started traveling, because I don't really have anything big planned on that particular road.
If you have nothing planned and don't intend for this travel to be engaging, skip straight to the point where you do have something interesting planned. There is no point dragging it out just to make it feel like they have traveled further.
Travel as part of the story
Somewhat of a minor step up on the previous category. Sometimes your travel serves as part of the story you are telling. You can use your narrative description of the terrain and significant landmarks to set the tone or give vital pieces of lore.
In these cases try to come up with at least one interest location, encounter or interaction that can occur along the way. Combine that with as many random encounters you feel you need (0-2 per day is a good guideline) to keep things interesting. I like to use this random encounter generator from donjon for ideas, but you can use whatever you want.
It is important to keep things moving in this situation, don't drag things out unnecessarily. Give the players and opportunity to engage, if they do that's great, if not, move on. The goal is to make getting from A to B interesting, if they aren't interested it becomes more like the first category.
Travel as part of the challenge
"It's a dangerous business, Frodo, going out your door. You step onto the road, and if you don't keep your feet, there's no knowing where you might be swept off to."
Sometimes the part of the danger and challenge is the travel itself. Either the road is dangerous, the destination uncertain or the party are racing time. When traveling is part of the challenge for your party, I find skill challenges are a great way to run this. Matt Colville has an excellent video on Skill Challenges, you can read my related answer for some more information on using them in 5e.
In particular Matt Matter, DM of Critical Role recently used a modified skill challenge as his party traveled through a dangerous swamp (SPOILERS AT LINK: S2 episode 93 "Misery Loves Company"). I say modified because instead of having a hard loss at a set number of failures Matt rolled for a random encounter each time they party failure a check in the skill challenge. It was a test of if they could reach their destination (3 successes) before the encounters drained them of their resource.
I think Matt's solution is a great one for travel through a dangerous region. Skill challenges are a great homebrew mechanic and you should modify them to suit your needs.
Treat the road as a really big dungeon. Plan out your encounters, traps and challenges the same as you would for any dungeon in your campaign. Instead of a 5ft scale, use miles or some other appropriate distance. Instead of being connected by narrow stone hallways and tunnels, your dungeon is connected by roadways and forest paths. Instead of secret doors, there are hidden game trails, spike traps become falling trees.
With clever planning and reskinning of the environment, your whole world is a dungeon and the players and none the wiser.
Make it fun
Whichever method you choose, I encourage you to watch and listen to your players. Ask them what they are enjoying and what they found boring and adapt accordingly. The goal of all these suggestions is to maximise the fun, either by making travel fun or encouraging you to skip it entirely.