This information comes from a recent email exchange between The Verge and Valve game designer Robin Walker. In addition to reaffirming the Half-Life: Alyx team's commitment to solo gameplay, Walker also said the title will be a relatively-straightforward, "one-way" adventure from start to finish.
While there may be scenarios where you can approach a situation from different angles, or take slightly different paths through an environment, Half-Life: Alyx is far from an open-world experience (in true Half-Life fashion). You'll also be able to save as little or as often as you like, which can't be taken for granted in today's age of always-online live services.
Walker also says the VR game was developed with the medium's limitations in mind. For example, the developers are aware that many players struggle to use VR devices for long periods of time, so many aspects of the game -- particularly relating to pacing and story -- were designed to accommodate shorter bursts of playtime. However, the team is still confident that most people will be enjoying their upcoming game for 2-3 hours per session.
Walker notes that "literally every" Valve playtester who tried the game ended up playing for longer than expected. Granted, the playtesters in question were equipped with Valve's high-end Index headset, which was designed with comfort in mind from the get-go. Things may not go quite as well for ordinary players, who could be stuck with older, bulkier, and decidedly more uncomfortable VR headset models.
If you have a VR headset and are looking forward to the return of the Half-Life franchise, you can pre-order Half-Life: Alyx on Steam for $53.99. It's expected to launch in March of 2020.