Ok, I went out yesterday. Didn't quite do 15 miles (my gps wasn't working for the first bit, but I reckon I did about 12/13 miles).
Went to Whippendell Woods, and hated every minute - way too hilly for my liking, had to get off and push at one point. Instead I went back along the tow-path and back to the Aquadrome. Much easier then the first time I went, I'm really liking that route now :-)