This winter I plan on just doing a whole bunch of LSD.
Personally, I hate the term Long Slow Distance because I think it’s very misunderstood. If by LSD, you mean good aerobic base training, then I don’t see a point where you’ll see diminishing returns. If you run high mileage really slow, you’ll still make gains but it will take longer and you’d probably be better off running fewer miles at a faster (but still aerobic) pace. 30 mpw isn’t that much, really. If there is a point of diminishing returns, I would guess that point is over 100 mpw. It’s tough to say, though, because a lot of that would depend on how much other training you’re doing and how you’re recovering. You may have to back off the cycling and swimming a bit to accommodate the increase in running volume.
Healthy Intelligent Training by Keith Livingstone is a really good book on run training. It’s based on Lydiard’s principles, which focus on aerobic base training, and I believe that’s where LSD came from although Lydiard always said his runners never ran slow.