The generally accepted wisdom in the running community (backed up by a couple of studies, but what isn't these days) is that HIIT is great for making big gains

The generally accepted wisdom in the running community (backed up by a couple of studies, but what isn't these days) is that HIIT is great for making big gains in the short term, but that the gains will peak before a program with proper high volume base training would. In fact, the way most people are encouraged to train is to build up to some decent mileage (at least 20 miles per week) and then mix in speed work, which varies from simply faster runs, to Fartlek (speed play), to intervals. Knowing this, it becomes obvious why runners fall in love with speed work: they might do a "traditional" training plan and run slow, high volume for 3 months, and see minimal but steady gains. Then they do 3 weeks of sprints and make huge gains! Awesome! But those gains were a reflection of the base work. Think of it like sharpening an old, abused, rusty knife: you don't go right for the fine grit. First, you grind it down, then successively apply finer grits. The knife won't cut a tomato without the fine grits, but if you skipped straight to them, you'd be at it forever. The other reason one should be wary of HIIT is the increased risk of injury. Especially in running, if you go from couch to sprinting, you're highly likely to be injured. It is important to run slow and often, so that your tendons, ligaments, and joints (not just your muscles and heart!) are ready for the strenuous demands of running. Similar concepts are often spoken of in weightlifting, where you are encouraged to start with just the bar and work up.