How can I switch to fartlek run if I do constant pace running 3/4 times a week & cover 2.5miles in 22mins? At what time interval should I switch pace?

intervals, sort of. The idea behind fartlek is to not get too ocd about the times or distance. Try running 880 mile walk 220 and repeat until your goal is reached. Push the pulse to target if you can with each leg.