Unless you have a limiter that makes faster running difficult or risky for you (e.g., Achilles tendon pain), you should swap out Foundation Runs. Whenever possible, you want to do your most challenging workouts in the sport you’re competing in.
Phil – I would do this in place of easier runs. In fact, that’s exactly what I’ve been doing since May. I was still in rehab for a foot injury when I started back to running. The PT suggested running 4 days a week and doing a lower impact form of cardio on the other days, just to make sure the foot has plenty of time to recover.
So I’ve done aqua jogging (when the temperature allows), cycling, and zero gravity running in place of weekly easy runs. Probably a different reason than you’re considering it, but it has worked really well for me. I hope your training goes well!