Stache: Groove intervals are a term I use for repeats of between 1k and 2k done at 6k-10k race pace (i.e 93-97% vVO2 max) with 40-50% recovery time between each.
They are slower than classic VO2 Max but faster than Daniel's Cruise Intervals. Basically they teach your body to deal more effectively with a slowly raising lactate environment like what happens in many of the races between 5k and HM. Other names you prbably have heard these called include: marathoner's speed work, 10k pace intervals, etc.
Typcial example may be: 6 x 1600 w 2:00-2:30 jog recovery
For a schedule in the 55-60 mile a week range here are some suggested workouts by type:
AT - Tempo: 7 miles
LT - Tempo: 4 miles
LT - Repeats: 3 x 3k w 2 min jog
Groove Intervals: 5 x 1600 w 2:00 jog
VO2 Max: 7 x 1000 w 2:30 jog
Fast Intervals: 16 x 300 w 1:30 jog
Easy Paced Long Run: 14 miles
Steady State Long Run: 12 miles
Progression Long Run: 12 miles
Race Week Cut-Down: 3k @ LT, 2k @ Groove, 1k @ VO2 Max, .5k @ Fast Interval with a 400 jog between each
On doubles: I love doubles but only once the primary easy run gets above 1 hour in length (except long runs). For most people this means starting to add in doubles once 60 miles per week has been reached.
For example, once your Day 3 and Day 4 runs are up to 60 minutes in duration I would keep them at 60 minutes and add in a easy 20 minute secondary run rather than lengthen the duration of those runs.