Just to add another data point to this discussion, I ran a half marathon (more than 2 and 3 years removed from run training) in 1:13 2 years in a row on 0 miles per week BUT I was biking on average 50 km/day to and from work (in the Summer) and my job required me to be on my feet all day. In the Winters, I did swim workouts every morning with some elite triathletes and ex-collegiate swimmers, which probably aided my VO2 max. In high school, I also entered a marathon on zero run training and ran 2:54, splitting 1:17 at the halfway point, before badly running out of gas and needing to take walk breaks in the second half.
I ran collegiate track and cross country for 2 years, as a walk-on, with no real running experience before joining and had some success, but never managed to stay injury-free for more than 3 months at a time. Eventually, injuries forced me into quitting and I took up swimming to stay in shape.
A few years after graduating, I started to do some run training on my own again and got down to 1:08 for the half and 30 min for 10k on pretty inconsistent training.
I've found that how quickly you adapt to a sport depends a lot on your natural body-type and background. I know that for me, if I don't swim for 6 months, I lose a ton of muscle and fitness, but other guys who have natural swimmer bodies and a long history in the pool, can get back in the water after an extended period away and not lose nearly as much. I don't think that there was ever a time (since high school) when I couldn't have easily broken 35 min for 10k if I wanted to, whether training or not.