Hi,
I run commute 5km each way, 5x days a week, but want to be training for longer ultramarathons (50-100milers). I know the best way to build endurance is by easy long runs, which I try to fit in on the weekends when I can, but my question is: how can I best make use of my 10 weekly run commutes? Should I run easy (zone 1) even though there's probably no benefit to the slow twitch muscles / aerobic system since it's only 20-25minutes? Should I run near lactate threshold or VO2 max every time, to build VO2 max for at least a higher aerobic potential? Mix it up? I've heard too much higher intensity running actually compromises your slow twitch muscle fibres?