Of course athletes of most countries could've obtained steroids relatively easily in the 1960's and it is more likely than not that it wasn't only the shot putters/weightlifters who abused them, but that the practice had spread also to endurance sports.
But still even when there are many proven cases and confessions of endurance athlete steroid use from the 1970's, there is not one proven case from the 1960's and even the speculation that it was widespread among endurance athletes is pretty nonexistent in the literature. It is possible that when the East German doping program started, mid D runners were also taking steroids from c:a 1968 onwards, but it could've been more a trial-and-error type experiment to give steroids to every athlete than based on any pre-existing knowledge about the benefits.
Even the magnitude of benefit is not that clear for men, because females (Semenya) benefit generally more from steroid use and I don't think that Lance attributed his success to testosterone which could be beneficial during intensive cycling season and during the multiweek stages races. It is telling that when one DDR doping doctor listed in 1977 how much athletes had improved significantly during four year steroid use, he listed various disciplines but didn't bother to mention men's middle distance at all whereas women's 800m and 1500m are mentioned.
I think that practically zero experts have given any estimate how steroids improve the perfomance of male endurance runners and I don't know the benefit and neither does anyone else, so it is tempting to select a figure to suit anyone's own narrative. As one interesting anecdote, one Finnish antidoping activist and steroid specialist told in 1985 that whereas every strength athlete and a majority of sprint athletes took steroids, the benefit on endurance athletes was "unlikely even when there does exist some random use". He had revealed earlier (based on interviewes with Finnish sports MDs, athletes and coaches) that a large portion of the 1976, 1980 and 1984 Finnish olympic teams had doped and the Martti Vainio steroid case had taken place only a year earlier. Clearly placebo in his view, which is an opinion just like any other one.