COVID-19-induced quarantine may lead to deleterious effects on health status as well as to impaired performance and increased injury risk when re-starting training after lockdown. We investigated the physical activity (PA) habits of recreational runners in Spain during a 48-day home quarantine during the COVID-19 pandemic and the characteristics of the first outdoor running session after confinement. A cross-sectional study, including a self-reported running questionnaire completed after the first outdoor running session after quarantine, was performed. Three hundred recreational runners (74% males; 60% 18–40 years old; most typical running experience >3 years, 10–30 km weekly running distance distributed in 3–4 sessions) were considered for analysis. Advanced runners ran, at least, 4 days/week and participated in running events. They performed significantly longer and more non-supervised weekly training sessions during confinement (p < 0.01 for both) than novice and amateur runners. Most runners performed their first outdoor running session on asphalt (65.3%) and ran 5 to 10 km (61%) at a pace above 5 min/km (60%), reporting no pain before (77%), during (64%), and 24 h after (76%) the session. Advanced runners performed a significantly longer running session, at a higher pace, and covered a greater distance (p < 0.01 for all) than novice and amateur runners, while enjoyment and motivation tended to be significantly higher when runners’ level increased (p < 0.05). Higher training levels prior to and during confinement may lower the collateral effects (e.g., detraining, injury risk) of home quarantine when runners return to previous PA levels.