High mileage day by day is a stimulus by itself. You’ll get sore walking 10-12 miles a day. I’m doing about 70-80 per week, I have a bit better race fitness than you, and I’m happy with 5:50s over hills in the heat. Don’t stress about it. Tempos are to train your heart/aerobic system to maintain a higher heart rate more efficiently. It can be discouraging, but effort is key. Some days I can manage 5:25 pace for a 4 mile tempo, sometimes I’m struggling to get under 6:00. As long as the effort is there you’ll improve.
If you are struggling with heat, get on a treadmill, set it to a 1% incline, and dial in the pace to the proper heart rate zone. It sucks, but you can get a really good, specific stimulus, and if you are like me and proper tempo pace/effort is rough to maintain, the treadmill forces you to maintain it.