How about starting out from Fenwick village- if you fancy a few hours run out? Whitelees is great if you like windmills but it is a bit boring just going there on it's own. There are some good views on a good day but no singletrack just stony fire roads however I tend to do a loop starting in the local trails at craufurdland then head up into whitelees thru waterslap which is a nice way in and I have always fancied going over the top of the eaglesham moor down to the old A77 and back to fenwick? All retro friendly trails and a good bit of country road so would be about 25-30ish miles all in? Pub at the end too.