Long, slow running provides the same benefit to "800/1500 runners" as it does to long distance runners. All long, slow running does is build up your aerobic capacities. The only difference between mid-D and LD training is the ratio of aerobic vs anerobic training needed to meet the demands of their respective events. LD events are almost entirely aerobic, so lots of aerobic training is required relative to MD events.
This is obviously very simplistic, but I hope I got my point across. To answer your question, do your easy mileage, you guessed it, EASY. To give you an idea of what this feels like - you should be able to carry on a conversation or atleast speak one sentence before needing to take another breath. Don't look at your watch, in fact, my training only got better after I stopped paying attention to my pace during my easy runs. Anyways, I hope this helps, good luck.