You could try using the Delete Homestretch feature, which lets you drag the finish point back along the route, however it can get confused when you have multiple laps on a route like the one you have. Otherwise you would need to replot the route.
John
https://www.plotaroute.com/map/441140 is a map for a USATF and NCAA cross country race. I added one too many loops. How do I remove a loop to turn this into a 4 kilometer course? How do I remove 2 loops to turn this into a 3 kilometer course? Thanks for your help. Blessings, A