Bike interval units question

The ' stands for minutes.  So 20x0.5' really means 20x30 seconds.  Another common abbreviation is the ", which would mean seconds.

So 2x20' would mean 2x20 minutes, or 20x45" would mean 20x45 seconds.

I don't think you'll often see intervals measured by distance.  This is because distance can largely be affected by wind or terrain.  You can do intervals on a hill or on a flat area...but 5 miles on a hill is going to be a lot harder than 5 miles on a flat area.  But with time...5 minutes is 5 minutes...no matter what terrain you are on.