I think that the fact that it takes time to bring levies from far off counties and that it varies by distance is a good thing, even if the time required isn't necessarily accurate. Besides, even if you got the speed down correctly, I don't think it was common for levies to immediately start marching as soon as called. At least not every time. You'd have to first get the message to assemble, which would mean someone traveling there in the first place, but even if you ignored that part, it might be harvest time and they need to deal with that first or something along those lines where they can't march for a week or two (or longer). And even if you ignore that aspect, it can take some time to get everyone ready and provisioned (even with minimal supplies) before they can march, adding at least a day or two delay. The game could add in all these things for realism, but I think just having a reasonable delay based on distance is enough without trying to get everything exactly right. 
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