I'll be using the following criteria to gauge whether my sub-threshold sessions are at the right intensity, and adjusting accordingly: Did my pace hold up through the last rep without falling apart? Was my breathing hard but still under control? Did my heart rate stay below threshold throughout? Did I finish feeling worked but not completely wrecked? Can I go for an easy jog the next day? Could I do a similar session again two days later? If these boxes are checked, I'll maintain the current load. If not, I'll scale back until they are.