Since you already ran a 5K I would use the average HR as your LT and set your zones using Friel’s heart rate zones.
From http://home.trainingpeaks.com/blog/article/joe-friel-s-quick-guide-to-setting-zones
It says to use the last 20 min of a all out 30 min run but I’ve found that a hard 5K is close enough.
Run Zones
Zone 1 Less than 85% of LTHR
Zone 2 85% to 89% of LTHR
Zone 3 90% to 94% of LTHR
Zone 4 95% to 99% of LTHR
Zone 5a 100% to 102% of LTHR
Zone 5b 103% to 106% of LTHR
Zone 5c More than 106% of LTHR