Not in the UK or an eternal optimist then?
I’m no expert, but from my dicking about you have a couple of basic options (1) use ambient light and a fast enough shutter to freeze the action or (2) use a flash (off camera if possible) on second curtain sync to freeze the action with a shutter speed to give an amount of motion blur (3) panning.
IMO it is much easier to convey a sense of dynamism if you get some blur in the shots, so I would either pan or use flash. Panning takes some practise; of course you can combine it with flash to give you a better chance of getting the rider sharp. The smaller the aperture you can use the more likely you have of getting something in focus, again totally IMO but you are better upping the ISO and having some noise than getting every shot out of focus.
If you’re in the trees then riders can be in and out of shade meaning the correct exposure will be hard to nail. I find manual easier, when you get it wrong it’s massively wrong, but on average I think I get more keepers.
Get the riders head sharp, fill the frame, take lots of shots.