In reply to Removed User: It may differ for you because of the limiting factor you identified, but when I introduced once weekly hill sprint or track sessions into my training on the back of otherwise very easy paced but reasonably high mileage, I found they rarely seemed to tax my cv system ( it differed for 800m+ efforts).
I'd find my legs being unable to accelerate and a lactic build up toward the end, but within a few seconds of pausing I'd have returned to relaxed breathing and my hr not overly high. So for me such sessions were useful, but for developing power and pace whilst having little impact on endurance. I think it is why tempo efforts hurt me more than interval type work, because only then do I really start to tax my cv system.