21k / Half Marathon Routes this is a darn good route if you want quiet shady lanes and uppy downy bits near Truro. This is not the official Truro Half route, which goes out south of Truro.
St Clement hill is a nasty final kick though. The link to Strava for more map detail is below.
Alternatively you could head straight over at the crossroads at around 11/12km, near Tregear. I turned right there to head alongside Tregasso woods, but if you carry on over the top signed Probus (road across arable land with no fence or hedges – bit strange in Cornwall) and hang a right for Tresillian later at the bridge, there’s a lovely long gentle climb and the routes rejoin at the junction. That detour gives the extra distance so you can stay on through Tresillian and turn off (after my route leaves the main road) onto either the Devil’s Arch road from Tresillian past Penair school and then pick up Penair lane on the right to return to start, or head up the main road, which has a pavement all the way to the park and ride, and cut through the P+R to start.