Why do you feel you couldn't plant the whole length yourself? You can get Rootrainer grown saplings which just need a small hole eased open to pop them in and you can take a few years over it. Mr F planted up a coppice and at least 100m of hedgerow as part of his recovery plan after cancer and major surgery. It took him a couple of years, starting at one a day then working up to a dozen or so each day over winter. If it's the cost, then bareroot saplings are surprisingly cheap, but need a bit more work.
However, I think just hoping a hedge will spring up of its own accord isn't going to work. Woodland will regenerate over time if there are trees in the vicinity to seed themselves, but the timescale is too long for your plan. For a nice dense hedge, with any chance of being stockproof, you have to plant it and protect it. We use tree guards, but we have never cleared the ground of weeds or used plastic mulch or weedkiller around the saplings - we have found that the fight to overwhelm the weeds seems to make the final trees stronger (as long as you pull out any rank growth around any struggling plants). You will also need a stockproof fence on both sides of the hedge, about 2-3 metres apart, to allow for the trees to bush out, and to keep grazers away.
When we first started planting up hedges, we began with 50m a year because that was all we could afford, and you can still see the age differences although nearly all are fully grown now.
Also remember that most livestock will pull up or demolish any trees and hedges they can get near, so hoping a hedge will be reliably stockproof is doomed to disappointment. You really have to do it properly, by planting, protecting and maintaining the trees.