Mainly flat (slightly uphill actually, but only 1-2% gradient) is the A323; starts pretty close to the University, through the Park Barn estate.
From the Black Bison pub http://theblackbison.co.uk/, heading West on the A323 – it’s downhill to start with but then flattens; then there’s a roundabout in about 1/2 mile (take the first exit straight ahead) where you might need to slow but visibility is usually Ok for you to pick a gap; after that it’s pretty flat as far as nearly Ash which has a set of traffic lights and a short hill thereafter.
It’s an A road so quite wide with good visibility but cars do come past quite quick, even in the bits which are a 30 limit. There’s also a right turn at Cobbett Hill Rd where in rush hour sometimes there’s a short queue and you have to pick past (or use the pavement!!) Used by lots of cyclists (me included) though so regular drivers of it are used to seeing and passing bikes.
I reckon it’s about 5 miles which maybe isn’t enough for a full 20, so instead you could start at Ash end, and use the roundabout as a turn and return course? Or there is a local ten mile TT course, that uses the same road but then a loop and which has a couple of small hills (up and then down, 95m total climb in the 10m)
http://www.mapmyride.com/gb/frimley-eng/charlottesville-10-tt-course-route-51551926