The Straight Course, as it's now known, was the fourth course introduced at the Regatta and started being used in 1924. It's a stretch of the River Thames from Temple Island to Henley Bridge and is one mile 550 yards long, so just over 2.1km in new money.
When the Henley Royal Regatta isn't on, it's still a wonderful stretch of river to walk. Cross over Henley Bridge and take the first left and keep left down a footpath and it will bring you out with the river on your left and Leander Club (ace of clubs) on your right).
You can walk, jog or cycle this flat stretch down to Temple Island and back and many folk do. Just be mindful that if rowers are training, give way to their coaches on their bikes as they're often more focused on the rowers than those on dry land!