I started these recently as well. I was amazed how difficult it was.
My plan said to do 1 hour of individual leg intervals. Ride until one leg is tired then swap.
I figured i could do 10 mins warmup, 20 mins each leg then 10 mins cooldown. Simples.
Did 10 mins warmup, unclipped one leg....lasted less than a minute before i had to stop
I cant understand how it's so difficult?
Anyway, i ended up doing 4 mins rest then 1 min each leg and repeating that.
It's great for improving pedaling technique, you can really feel the dead spots when 1 leg is working on it's own. Keep at them.