Good reminder LastStandards.
Can I suggest this website as a good ready reckoner - I've put a simple route in this link (that happens to be a smidge over 300nm and visits two inexpensive airfields that both do a good lunch), but you can change it and play all you like.
Great Circle Mapper
Alternately, SkyDemonLight is more capable, also free, and a good way to play around with routes (he says playing with it for the weekend's flying at the moment). It only really runs on Internet Explorer, to save you working that out for yourself.
G