The best way to complete marathon isn't just back-to-back tetrises for the whole run. It can be done in 193 lines by burning a double at the end of level 3, a single at the end of level 5, and a double (or 2 singles, costing line clear delay) at the end of level 10.

So the run would go 3 tetrises, a double, 4 tetrises, a single, 18 tetrises, a double, then 22 tetrises. Unfortunately, maintaining b2b in high gravity presents a bit of a challenge, but not an impossible one.