It is possible to theorise that Willoughby's encounter with Marianne might not have been the haphazard knightly rescue that it seems, and that there is a carefully constructed 'back-story'. The following chronology is not at all apparent when you read the novel straight through.
Brandon in Chapter 31 recounts how Eliza as a child would frequently visit him at Delaford. There was local gossip that Eliza was Brandon's love child. So it is entirely possible that Willoughby, who had been visiting Allenham every year, had been aware of Eliza, and her connection to Brandon, for a very long time, maybe even years. It is even possible they encountered each other, at least forming a talking acquaintance, and maybe Willoughby marked her as a future conquest if the right circumstances were to arise. It is even possible that Brandon and Willoughby crossed paths during that time, and Willoughby developed an antipathy for Brandon based on Brandon's reputation as an impeccable gentleman.
Skip ahead. Eliza disappears in February while staying with her friend's father in Bath (is that the opportunity Willoughby has been awaiting?) and is missing for eight months. During that eight months, it is clear that it is Willoughby who takes Eliza with him to some place and leaves her pregnant, then deserts her (perhaps as soon as she advises him that she is pregnant), without ever having told her his address (so Brandon reports in Chapter 31). If that is in fact true, and is not a fib told by Eliza to forestall a confrontation between Brandon and Willoughby, then this means that Willoughby had always kept the door open to doing exactly what he did, which was to abandon Eliza if she became pregnant. This is a damning bit of evidence of his mind-set, because it would otherwise have been the most natural thing in the world for him to tell her where he was from, and to learn that she in fact had visited her uncle there many times during her youth. If he did not do that, it means he has been up to no good from the first moment.
In Chapter 6, the Dashwoods arrive at Barton Park in early September, while Eliza is still missing. Within a day or two after their arrival, Sir John makes their acquaintance. We then read ‘and as he attended them to the drawing room [he] repeated to the young ladies the concern which the same subject had drawn from him the day before, at being unable to get any smart young men to meet them’. They would see, he said, only one gentleman there beside himself, a particular friend who was staying at the park, but who was neither very young nor very gay. He hoped they would all excuse the smallness of the party and could assure them it should never happen so again. He had been to several families that morning in hopes of procuring some addition to their engagements.
Probably Allenham was included in Sir John's local canvass that day. If so, may we speculate that Willoughby, who has just arrived back in the country for his annual visit (coinciding, it seems, with his abandonment of the pregnant Eliza), has heard Sir John talking up the Dashwood girls, and is ready for his next conquest?
Possibly he craftily watches; he waits for an opportunity to make a grand entrance. In Chapter 9, the Dashwoods are well settled at Barton (presumably about September). The girls in one of their earliest walks had discovered an ancient respectable-looking mansion. The whole country about them abounded in beautiful walks. So it was not the first time Marianne and Margaret had walked where Willoughby found her when they met, and he may have had occasions to observe her from a distance while out riding. He was only a few yards from her when the accident happened.