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Replacing 15mpw of running for 50mpw of cycling won't do it.
My thought is the only way you COULD see this benefit is too add a very aggressive cycling regiment. In that case, the total cardio benefit could outweigh the loss of specific run-training. But you'd have to train harder than I think you're comfortable with.
I think there's a point where you could pick up a little extra cardo, and then once you resume running you sort of leverage that into running.
If anything your biggest weakness at 5km is not enough experience. Learning the distance/specific pain is probably a bigger limit than pure cardio.
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