ok bear with me I have not ridden there for about 5 years
Go (ride) out of Marlborough on the HackpenHill rd as you go past the common and drop down the hill on the S bends there is a track on the r/h side you cannot miss it this goes up to Barbury castle
Get to the castle and go over the top of it
then cross the road and follow the track to Hackpen Hill
Cross the road and follow the track for about 3 miles when you come to a crossroads in the track right is to Avebury, go left over the fields to Fifield Down follow that track and it takes you back to Marlborough
Not the best of riding but it is easily accessable 15 mile loop
btw there is a club in swindon that would be worth a look they appear to be doing good things to the Croft Woods
http://www.mbswindon.co.uk/club/