I actually did this. The result: All the gains from easy-running were negated, and I ran about the same.
If anyone wants to hear the extended story, ask, but otherwise, here's the summary:
~ I picked up running in my late 20's.
~ For 4 years I never went farther than 8 miles on any run, and only did singles. About 45 mpw during that 4th year.
~ Then I started exploring easy running and long runs. I got addicted to the easiness of easy runs, and did only easy runs for 8 months straight. 60 to 70 mpw during this time.
~ Then I entered a '5k' that was about a tenth of a mile short, and I ran 25 seconds faster than my PR for a true 5k, so about the same time (if not slightly slower) if the courses had been the same.
After being very disappointed that I put in 15 to 25 mpw more for 8 months and got pretty much zero benefit, I changed things up, but got injured. Still recovering from that.
There's no way I didn't get any benefit while doing the easy running, since I had never done truly easy running, nor long runs, prior. However, with no time improvement for 5k, I can only reason that any gains were neutralized by lack of faster training.