I would just increase their general work like you said. Last year I had a similar problem but on my low days I was only doing something like 12x100m. This year I would do a longer warmup + 15x100m + ab circuit + gs circuit or bb circuit.
I find that caffeine/stimulant intake and lack of sleep are more detrimental than additional general work in regards to fatigue going into high intensity sessions.
Discussing nutrition can sometimes be a tough issue. Most girls end up thinking that they have to modify their diet to eat a lot less when in fact they can eat more as long as they replace their junk food choices with healthy alternatives. I am big on zig zag dieting or whatever you may call it. Eat larger amounts on high workload days and less on lower workload or rest days.
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