Search Veronique Billat training, she wrote a famous academic paper on this.
She recommends to start with 30/30 and then progress to 60/30 with the 60 fast and the 30 at tempo pace !
I found this on the web:
30-30s (version 1) . The result, now known as Billat 30-30s, involves alternating between 30-second surges at vVO2max, and 30-second jog recoveries, and doing it for as long as possible. What’s interesting about this workout is that during the recoveries, your metabolism is still ramped up to VO2max for a significant amount of time. And it’s still high enough at the end of the recovery that it quickly returns to VO2max when you speed back up. With these quick alternations, she found, you can spend nearly 40 percent more time with your metabolism at VO2max than you spend actually running at vVO2max. But it’s not as taxing as it sounds. Billat found this to be an extremely good early-season workout, and also good for relative newcomers to speed training(6,8). Fifteen to twenty minutes of this, is a lot for most people, so luckily it’s also a workout that’s over fairly quickly!
30-30s (version 2). Still later, Billat developed another protocol, this one a (very fast) fartlek session. This workout is the most intense of the three, being suitable mostly for fairly experienced runners, late in the season, rather than early on. After warm-up, you pick up your pace to vVO2max, and hold it for a minute. The reason for 60 seconds, rather than 30, is to not waste time ramping up your metabolism. Also, if you are running on a track, this will also allow you to cover at least 200 meters, a good point at which to take a split to make sure you’re not wildly off pace. After that first fast-paced minute, you cut back not to an easy jog, but to tempo pace (perhaps 45 to 60 sec/mile slower than vVO2max) for 30 seconds. Then you speed back up, repeating tempo-pace recoveries and 30-second surges for as long as you can.