I run steady for an hour and then do two miles, one quarter on and one off. The on is 5K race pace, and the off is quicker than steady but enough of a break. I find that this is a bridge between interval quarters at faster than goal pace and race-pace simulators. I personally prefer all this to tempo which, given the specificity of training, merely trains one to race at tempo. Of course, one isn't bound to quarters, nor the track. Having grown up in Eugene, I guess the concept of Dellinger's 30/40s is ingrained. I think switching gears, with rapid recovery, when one is already tired really builds fitness and has a useful neuromuscular component. My advice is to run the offs at a pace that doesn't feel like a lot of recovery. Otherwise, you are just running intervals.