just to add on to what others are saying really. it's do-able in theory but i doubt it would be a nice break, mostly travelling etc, rushing round the sites etc, doesnt do any one of them justice. and as others have pointed out, take into account ferry times, trafffic, (tractors can really take up your time on windy roads if you are behind one!!), the pace of life, hold ups and general stops etc.
you say you are in Liverpool, well i agree with moth why not do Cumbria another day / weekend / long weekend? the sites are easy to get to from liverpool, and easily accessible.
we thought of the same thing as you a coupla years ago but soon gave up on the idea, orkney will be a time all on its own, especially as the times to visit maes howe etc are either early morning or late eveneing when all the americam tourists are gone back on their cruises or whatever and the sites are less ''touristy'. also once you get a feel for the place there are so many wonderful sites and 'cheery' locals who will direct or even take you to sites/ trips on outer islands. orkney is a place for giving it some time, and will be awesome!
also the expense would be horrendous for the length of time (not alot) you would spend, ferries etc are at times expensive (we found) when you add it all together with petrol and travelling and sleeping and eating.
lewis is also a place in itself and to just flying visit callanish would be a loss as the other sites and the landscape around is so very important.
whatever you do enjoy! reember the midges are out on the west coast (although orkney shouldnt be too bad) june - september,
but have a good one whatever!
love
pixie
xx