TheOgDonaldTrump wrote:
Seriously. Whenever I attempt a tempo I always bomb the pace. To play it safe I added 50 frickin seconds above my 5k pace per mile but I still struggled. I did 1.5 miles at said pace and I was pretty much dead. I didn’t want to quit so I tried to maintain that pace but failed and came through at a whole minute and 10 seconds slower than my projected pace for the last 1ish mile. I’ve done progression runs, mile repeats, 800s etc at way faster than my tempo pace and I always kill those workouts so I don’t what the deal is. My diet and sleep has been sound.
Out of curiosity, what exact minutes per mile paces are you running? For me, 5K pace plus 50-55 seconds has consistently been marathon pace. That was true when 5:10 was 5K pace, and it's true now when 6:10 is 5K pace.
So, your 5K + :50 pace should be approaching marathon pace. Surely you can maintain that for 3 miles. Are you estimating your 5K pace (and tempo pace) on the 2-mile time trial you mentioned? Was it an accurate 2 miles? Any recent races at all?
If you were running less than 20 mpw, I would suppose it to simply be underdeveloped endurance. But at 40-50 mpw, that shouldn't be the case (not too much, anyway ... that has generally been my mileage).
As has been suggested, I would start with 1-mile repeats (3) at perceived tempo pace -- not checking splits -- and taking 2-minute recoveries. The next week progress that to 1.5-mile repeats. Then 2-mile repeats, then try a 4-mile tempo run.
Good luck.