No, it definitely calculates mileage regardless of race or practice or whatever. Perhaps it is done at the split time points, I'm not sure. I do know that you can clock up as many miles as you want without finishing a lap (pit before end of lap etc).
yes, even if you'd go in circles all the time, LFS would count and report mileage. But any stats are only recorded on licensed hosts.
Anyway, looking at lionhead333's stats, I see 83 miles in total, so it's actually ok? Or did you do those laps after you posted this thread? (hm looks like it)
Mileage isn't counted one metre at a time by LFS World. LFS buffers those numbers and LFS World only gets them with intervals, so if you're online and drove a while, it's not always immediately represented at LFS World (mileage and fuel stats are the only things that work buffered like that - all other stats are pretty much realtime (2-3 seconds delay)).