I just started STS for the second time around, and I'm trying to work in more leg work by doing something like this:
Day 1: STS upper
Day 2: STS lower
Day 3: STS upper
Day 4: cardio that's leg intensive (someone suggested the all blast premix from low max for instance, or kickboxing would be good probably)
Day 5: barre (and I added kettlebells for some cardio)
Day 6: run
Day 7: yoga/stretch
It's a little odd with the front-loaded STS and back-loaded cardio, and I've only just finished week 1, so I may decide I don't like it, but for this first week I really liked doing it this way.