I just recently (in the last 10 years) started working on double bass seriously, and so far, what I found was the trickiest thing was sifting through the many different technique ideas to find one that suited me.
In the past 5 years, I have settled on a heel up, floating foot kind of thing, and then just do what you do, where you pick a tempo range, and work on endurance within the range. You are at a much faster tempo range than I am right now, but my range is getting better.
Growing up, I totally lived in the world of fast playing, but was concentrating on getting my prog/odd time sig chops going when every one else was blasting speed. I was playing to Fates Warning while every one else was playing to Slayer...I feel like I need to catch up now