I've always ran single days, usually 1 tempo run and one intervale session per week, with the other 5 days being easy runs at 7:00ish pace. Ran 17:11 in 2017 and barely improved down to a 16:27 this past January So that's little over 40 seconds off my 5K time in over 2 years. Then I started adding double days to my training, doing a 2nd run 5 days per week at a VERY slow pae (8:00-9:00 mileage) Since then, in just 5 months my 5k time has plummeted like never before. TT'd a 15:42 today. That's an improvement of 45 seconds, despite having no competition to help me push the pace like I had in January. If you told me in January that I'd be comfortably sub-16 by June I'd have laughed in your face. The only change I made was adding some super slow evening runs, kept everything else the same. I think the idea that "Really slow miles are just burning calories" is very misleading. If I'd not foolishly believed this logic for the past few years, who knows how much faster I could be today?