How do you implement sightseeing, shown for example in the North Downs table (p. 197), into what is, presumably, a journey?
The table on 197 provides random things you might find while exploring a ruin. Therefore I wouldn't necessarily say that's part of a journey. But maybe you tell them "As you walk along, you notice some crumbling ruins on a hill to the left." Being players, they get excited and say, "Yeah! We go explore them!" Now what? You didn't prepare for this. Well, roll on the table to get things started.
There are several tables like you're talking about. You can use them to flesh out Events that occur as a result of the Journey rules. (See page 114 for describing Journey Events.) In that case, you might want to pick something appropriate rather than a random roll.
As the Lore Master, you can throw them in any time you feel like it too. Suppose nothing interesting has happened for a while. You notice the players are looking at their phones or stacking dice. Probably time to shake things up. This is known as the "Send in the Ninjas!" technique.
More specifically: how do you count time during exploration, do you allow short and long rests, do you treat the table event like a Landmark (possibly hanging around for a couple of days), etc.?
Try not to let the rules push you around too much. Remember
they work for
you, not the other way around. Most things are obvious and don't require rules to decide. Logic and creativity are enough. When they aren't, you have the rules. (See "When to Roll" on page 16.)