When we venture down to our local track and prepare for our evening interval workout, we know what the workout is going to be down to how many intervals we are going to run, how fast, and how much...
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Option for randomization I've used when working out alone (no coach or running buddies to insert external randomness). Throw a dice and run the distance times 100 (for speed; equal rest) or 1000 (for tempo intervals; rest 20 % of interval time or whatever). Or listen to music and change paces every song (it is really hard to estimate which pace is appropriate for any given song). Lots of ways to get some good uncertainty when you get creative. :)