So yesterday I jogged 2.5 miles (4km) at a pace of around 6mp/h (10km/h). It was a decent run at a fun pace, not working myself too much (was barely out of breath). My friend however said that running at that slow a speed barely has an effect on your cardio. In order to start getting a cardio workout you need to be at least over 8mp/h (13km/h). Is this true? I know its obvious that the faster you run the better the workout is but is it true that my initial "slow" speed barely has an effect and is basically useless?

Tell your friend to fuck off.

Cardio kills gains.

pretty sure mph does not have a " / " in it

and uh, so i hear, its heart rate that matters

are u trying to loose weight? or condition yourself for endurance running? or what other goal?

endurance with the least amount of gainz loss

You need to work out just below your anaerobic threshold I believe for best aerobic results

u wanna do hiit then

well you're doing almost the exact opposite. the most efficient way to do what you've described is High intensity interval training

what u want to do is lift fast. that is good cardio