Yeah, i am big about associating my definition of tempo run with perceived effort. I usually think of the discomfort level as tedious. It's uncomfortable enough that you dont just feel like you could go forever. Still enjoyable though in my opinion.
But just an example: I remember last fall i was gearing up for a half marathon. I hadnt had my eye on it for two long but i had just graduated from college the spring before and i was still riding out my fitness.
I had been doing longer tempos and then about 3 weeks before the half i did a 10k race at an 85 percent effort. I ran 34 flat. A week and a half later I wanted to do an 8 mile tempo run about 10 days before my half.
I got out for the run that day, and I guess i was still feeling some fatigue from the 10k or something, but i just went with my definition of what my tempo run pace should be. In any case, if my memory serves me well i think my miles went something like this: 6:27, 6:20, 6:16, 6:16, 6:15, 6:14, 6:12, 6:05. Something like that. In any case, on the one hand i was thinking i went pretty slow and that worried me. In the months before i had gotten down pretty easily to 5:40 pace or even 5:35 at the end of a 6 mile tempo. And that day 6:15 pace felt just about right.
But i was smart about it. You can't make your tempo into a pace. you are going to get out of it what you should get out of it if you go that particular uncomfortable pace.
In any case 10 days later. I ran 5:39 pace for a 1:14 flat half marathon. Started around 5:45 pace and worked down to 5:32 by the end. I really did just need to recover that week before and i let myself.
So i have taken what i learned from that race to heart, and i always start out my 6/ 8/ 10 mile tempo runs with only perceived effort in mind/ no pace goal at least for the first mile or two. With a 4 mile tempo run i might shoot for something more specific though. But that might just be because i am less familiar with how my body should feel at the faster tempo pace