The DMG has suggestions for campaigns on the scale of province, kingdom, and continent. When you take into account the seeming rarity of going to levels beyond 12 or so (from the statistics and personal anecdotes I read), it looks like kingdom scale is usually enough even for long campaigns. Have any of you ran or played in a continent (or more) spanning campaign? If so, what considerations did you or your DM need to take into account to make it work? Did the levels come slower, for instance? Did distances between locations need to be tracked more carefully, or could you play fast and loose with them without anything going haywire? What did you enjoy most about them that wasn’t there for smaller scopes? 

We did a continent level campaign in a DragonLance campaign back in 3.5E, but mainly it was set pieces, we didn't have to game traveling. We arrived where we were needed and did the adventure/module.

I think in a bigger scale campaign, you have to either gloss over, or pay extra attention to, the traveling aspect of the adventure.

I'm in a campaign that's getting ready to wrap up (we're level 19-20) after about 5 years. We have a wizard, a druid, and a ring of reliable teleport, and even before that we had a ship, so we had a fair amount of continent-hopping and travel. The campaign was XP instead of milestone, which I do think meant it took longer to level up than milestone would have. We did some amount of estimating of how long travel would take, but we tended not to roleplay the whole travel, just skim over it, with maybe one event happening per day, if that. I've played in other campaigns where that matterd more, though. It really comes down to what aspects your group is interested in.

I tend to run continent sized campaigns (if not larger). Typically I start in the early levels by having a lot of focus on travel - this gives players an idea of the world’s scale, while also giving low-risk encounters to learn character mechanics and plenty of opportunities for players to roleplay and get to know one another.

As the campaign advances, I increasingly shorten the travel - it is not as necessary for the above goals and starts to feel repetitive.  It might come back in more detail if they are travelling something particularly dangerous, but mostly is going to be hand waived by level 13+.

does distance matter at all?

I would think only if your campaign involves a lot of day to day travel across country.  

One does not even need borders.  Just put a location on a piece of paper and a destination and say, this is x number of distance units.  For that matter does one even need a map?

I suppose if your party wants to make decisions on where they are heading. Like in the Tomb of Annilihation.

However, that being said my world is around 50,000,000 square miles in size.

I run an FR campaign that was originally established as a place to retire my epic/tier 4 characters from 1-3.xe. Even for them it started as a (sort of) smaller campaign. The mages and clerics took a year at CANDLEKEEP to learn about the history and people and groups of the world. Meanwhile the rogues, fighters, Druid etc split into teams and hired onto caravans to different regions to learn about the routes and trade goods as well as potential hazards of the world.  They started as cross continent merchants running a caravan from Waterdeep to Cormyr and the Dales. In the Dales they destroyed a green dracolich taking over its lair as a hidden base then quietly exploring Hillsfar, yulash, Zhentil Keep and Teshendale/Daggerdale. They built up a base of colonists from retiring adventurers, caravan guards, and displaced freemen from warring regions while running caravans between the Dales, Cormyr, Waterdeep, and Calimshan. When they were ready they took Teshendale (and Daggerdale) and brought their colonists in standing off the Zhentish and Hillsfarian armies.  They took on the Zhentish and ended up taking over Zhentil keep then got involved fighting with the  Phaerimm, the Shades and the Fey’ri. They built an alliance with the returned elves, Cormyr and themselves. They added new followers and were exploring the Feywild, Anauroch, the Imaskari ruins and the Shadowfell. They continued through the 4e period and ended up defeating the Drow beneath Cormanthor, the shades and Shar ( defeat not destroy, and a homebrew “undead 4th child of hers - an improved Atropal Scion”) they are now primary NPCs in the ongoing campaign with fingers in everything from the Dales and Moonsea to the sword coast to the great south Sea and the Golden Way. That’s really sort of what happens with epic characters - they become epic NPCs in an ongoing world. ( That or they and their world are simply retired and discarded).

I'm running a modern day zombie apocalypse campaign right now that will very soon become continent-spanning. That being said, the continent is North America and the characters have access to cars (though with very limited resources and many pitstops along the way), so travel is either day-long stretches with a roll for a random encounter/weather event and time for players to roleplay along the way if they want, or just skipped over and tallied as distance traveled. They have access to radio, and allies in certain places all over the continent should they reach out, due to the connections a couple of characters have.

With the technology in this world, and the fact that the distances between locations are real, actual places with distances i can look up, i could plant fun locations and random stuff and know exactly when the party is going to reach it. Leveling isn't really effected, i just sense if the campaign is slowing down a bit, find something exciting and scary to happen, and level them up before it happens so they're prepared, then throw it at 'em. This seems to be cycling at around every 2-4 sessions but will probably slow down a bit as they hit second tier. 

Having a party with specific locations they know (or at least think...) they'll be safe at, with danger in between has been a great setup so far and i only see it improving. There is a gradient of danger to the map that corresponds to how the story and levels will progress as characters find out more and travel farther in the Dangerous Direction (Northwest, in this case), and that will stay consistent even as they level up, so they can travel backwards to find safer havens. Planning a campaign off of just, Earth, has been so easy and fun, and if you have an in-depth enough world built, i'm sure the same could be said.

I am currently running 2 different campaigns, with a total of around 12 players. 
The adventure is starting on a singular large continent, but due to the nature of the game I expect a fair few of the players will set out to explore, and then discover various others. Though something that is different in this aspect, is that the world is a full sandbox and fully pbp, so each players has the freedom to do what they wish, and when they wish to. So whilst normally a party might be more focused on going city to city to go after the main plothook, this allows for everyone to focus on their preferred aspects of dnd.

In normal 4-6 player session based parties I wouldn't go for the same type of play, unless the party was set out on it.

TLDR: Continent spanning campaigns work best with a full sandbox and alot of freedom for each party member. If going across continents happens with a party, I'd personally make that one of the plotpoints, instead of being something that just happens.