This phrase is a metaphoric hyperbole for a short time. The actual time was months, not hours. Alternate translation: "short time" (See: [[rc://en/ta/man/jit/figs-metaphor]] and [[rc://en/ta/man/jit/figs-hyperbole]])
Here "heart" represents thoughts and emotions. Though Paul and those traveling with him were not present physically in Thessalonica, they continued to care and think about the believers there. Alternate translation: "in person, but we continued to think about you" (See: [[rc://en/ta/man/jit/figs-metonymy]])