My hamstring appears to have strengthened over the weekend (didn't climb this past weekend).
R 1.6m 25:00 3.8-4.0mph 145-155bpm
5 min break
R 1.0 15:30 3.8-4.2mph 145-157bpm
I found that I could last longer if I made myself try to keep my heart rate at 150, hence the slow down to 3.8 after running 4.0 or 4.2 initially. If I slowed down ever so slightly I could keep my HR a bit lower.