Either works. Your body doesn't know the difference. There's no magic percentage or number of minutes. Think of it this way: if you are going to run a 3+ hour marathon, as opposed to 2+, (or a 45 minute 10k, not sub 30) you will be running longer, so why not do a 30 minute tempo instead of 20? If it's too hard a pace, start a little slower. My tempos are arbitrary. One happens to be a 2.86 mile Strava segment I discovered on a nearby bike trail, run at 10k, or HM, or marathon pace, depending on how tired my legs are. Sometimes I just go on a longer run, between 7 and 13 miles, and gradually crank down the pace to marathon goal pace or faster in the last few miles. The constant obsessing about formulas and HR percentages is silly. Just run. I'm 60 years old and just PRed in the marathon in October in 2:58. I run sub 19 for 5k with no specific training at that pace. I just run, easy most days, harder some days, a few miles most weeks pretty quick. Consistent daily training, at a variety of paces, not too many miles on the red line, about 10 percent less mileage than what would make you injured or chronically tired, that's all most people need to know. Just run.