Yes, of course she wasn’t ‘just passing’. But a woman of the landowning classes of that period, even an unconventional one like Lister, wouldn’t be walking any distance, she’d be on horseback or driven in a carriage. Showing up after walking a few miles along the roads would be as strange as dropping in ‘casually’ by helicopter
Walking was for peasants and tramps, quite apart from the fact that walking in UK weather on unpaved rural roads was likely to leave long skirts filthy and unsuitable for social visits. Remember the brouhaha Lizzy Bennett causes when she unconventionally walks a few miles from Longbourn to Netherfield and shows up looking flushed, untidy and muddy.
That’s why I wondered why we’d seen her shoot her horse — was it to establish she’s actually quite ‘poor’ before she starts exploiting her coal/Miss Walker’s fortune, hence no other mounts..?