Your comment on another answer implied you wanted to hear more about play style, pacing, and social dynamics, so I'll take a (long, it turns out) stab at that.
Spend a lot of time with the both of you discussing what the character wants to accomplish. Continually checking in and talking about the character's (and player's) objectives during the game will naturally help with pacing and allow you to keep an eye on the big picture, while highlighting the places in that big picture that you both find interesting to zoom in on.
Just as importantly, keeping the big picture and those goals in mind will put the focus of the game on the character, and it will help you, as GM, to concentrate on playing a supporting role to the character's unfolding story. The best part of being the player in a 1-on-1 game is being able to really dig into a character and push for their goals, so enabling this is a high priority.
Although pacing will probably come naturally from the big picture, if you expect to jump around a lot that will help you make the shift from a party-based style of play to a 1-on-1 style. Rather than allowing the game to move moment-to-moment (which gives a group of players lots of opportunities to all contribute), take the freedom of only handling one PC as an opportunity to hone your scene-framing skills. Tightly focus in on the points of conflict and character development (which won't always be the same thing) so that you can quickly get to what the meat of the character's story is right at that moment.
You might worry that such tight scene framing might miss out on those great "slice of life" parts of roleplaying that emerge organically when you play with looser, moment-to-moment scene framing, but you can have it both ways. Frame focused scenes that are deliberately about letting the character unfold, with or without a conflict. Put them in their home for a few minutes (for example), ask the player, "What's it look like when you first walk into your own home?" Let the player explore their character and interact with their environment for just a bit, to enjoy the experience. Such focused scene framing is possible without feeling artificial because in a 1-on-1 game you'll both get to know the character very well, quickly, allowing you to frame scenes that are just "obviously" interesting and relevant.
Other answers have mentioned that 1-on-1 games can be exhausting and you can get a lot done in them. Use variations in your pacing to build in breathing space where you can both relax and recuperate from the more intense parts of the game. After a tough scene, pull back and look at the big picture for a bit. Don't be afraid to let the game sort of meander occasionally just to get a break from such high-impact scenes. Make a point to pull out of roleplaying mode and chat about what just happened after particularly affecting stretches of game. While this would arguably be "wasting time" in a group game, you'll find that you cover so much ground in a 1-on-1 game that not only will you have time for it, but it will sometimes be necessary to properly appreciate what just developed, let alone think about potential repercussions.