I think this too. He knew he had to deny setting foot in the park so he said they had sex on Oak Road to distance himself from the river scenario. In reality, he could have had approx 3 minutes to get himself and Libby up to the pond before raping her and her screaming. That went on for a couple more minutes before he then legged it back to his car.
Now, did he kill her (and personally I think she was smothered during the attack, to silence her) and throw her in the river on visit 2, or did he leave her near the bank and jog up there to put her in on visit 3?
Either or, if you question the location near the green shed, an alternative location nearer the water a) makes the timescales more credible, and b) fits better with SA’s testimony.