Thinking of dropping the tempo run

As a relatively new runner it doesn't seem out of the ordinary to me for you to run 38 on tired legs in a marathon build up and then 37 on lower mileage. It could be that you have had time to recover from and benefit fully from your marathon block.

That said I am sure the intervals helped too. If you can handle 70 miles a week with a tempo you can probably handle 45 miles a week with intervals and a tempo. Swapping the tempo for a longer run might be good too but I would still do the tempo some weeks as well you could even do your tempo as part of the longer run. This is how I usually do my tempos if I find I am more tired or am having a down week or tapering I will just do a steady longer run.

You need a little bit of everything in your training--it sounds like you could just use more variation in your plan. Tempos to improve your lactate threshold, intervals for VO2 max development, sprints for muscle recruitment, and some sort of shorter intervals for rhythm work. You adjust the emphasis of each of these based on your goal race.

A simple but effective plan for anything under a half marathon has you picking 2 of the following workouts each week and the cycling through them.

6-8 x 1000 @ 10k (1-3 minutes break)

8-12 x 400 @ 3k/5k (1-3 minutes break)

8-12 x 200 @ mile (2-4 minutes break)

10 x 10 seconds gradual hill sprint (3-5 minute break)

4-6 mile lactate threshold run (0-1 minute break)

6-8 mile progression run (No break)

You had fresh legs after cutting the mileage, but still had some of the strength from your marathon buildup and long tempos. If you keep up this low mileage you will lose more of the strength and not only run worse in the 10k but your marathon would be terrible. But I get it, longer efforts are tough, 8x1200 is more fun and not hard for very long.

As some have said before. You have just capitalized on the prior work performed during the marathon block. Had to that the work done at around race pace with intervals and you have a plausible explanation. Intervals adaptation is short lived and the compensation from running lower miles will fade off too. You can use a a mix of Training means at the same time or perhaps cycle again between more mileage/tempo-lower mileage/intervals blocks. Just a matter of preference and time/life requirements. What you shouldn't do is think you only need one ingredient to bake the cake.