"Tune-up races are important benchmarks for your fitness and prepare you mentally for the rigors of racing. Because less is at stake, even the toughest workout isn't as mentally demanding as a race. .. The all-out aspect of racing provides a mental hardening that's necessary to run a good marathon," according to Pete Pfitzinger and Scott Douglas in Advanced Marathoning.
I even had a mini-taper, running only 21K in the past couple of days - both sessions at recovery pace. Thursday's had 6 X 100m striders. I am looking forward to a hard effort.