When one looks at the anecdotes, the somewhat prevalent use, possible mechanism(s) and the scientific research, the confluence of evidence points of towards some possible benefit. But the boost is likely so unreliable and modest that there is and was a huge overlap between athletes, who took steroids, and those, who didn't:
- There is just by a magnitude less endurance athletes, who have known to have taken steroids vs. other athletes, ergo likely the consensus view about the benefit was ambiguous at best. I don't know about any endurance runners, who admitted or otherwise we know having taken steroids in the 1960s, when they were quite readily available. IIRC, some Dutch cyclists used steroids for the 1968 Mexico City Olympics under the supervision of their team doctors.
- None of the few male endurance athletes, who used steroids, said that the benefit was enormous even if they felt some modest benefit in mood and muscle recovery. A few felt steroids had a detrimental effect.