Have been a runner for about twenty years. I have been last once or twice, been first once or twice.
My salad days are long gone, and now I do what PPs say - check previous years' results, and, if not sure, drop the organisers a line. I actually did do this once and the guy running it - good enough to win local marathons - said he'd be sure to keep the finish open for me and not to worry.
My take on the cheering and clapping is different from some PPs.
So many times I've been chatting to the pointy-end runners before or after a race and they've pulled out their phone and showed me a photo of Tweedle-Dee and gone, "that was me five years ago". So many times I've seen people who have won stuff in the past, now shuffling round to beat the cut-off, or, unable to run at all, now marshalling races.
So, the people cheering on the back of the pack, they have either been there themselves, or have friends who have been, or know that they are just a pothole away from being there.
I don't think it's patronizing or condescending. It is genuine fellow feeling and the desire to see all, regardless of genetic gift or current circumstances, do as well as they can.
I agree it's considerate to check - particularly in bad weather! - that you're not going to be hugely behind everyone else, and if you're dead last it's polite to at least look like you're putting in a mildly respectable effort - i.e. probably don't stop to chat or take photos unless you're at least last-but-one ;) But really that's all!
Lastly - as I said, a lot of us have seen potential emerge from quite unprepossessing starts, so, so many times over the years - so, if experienced runners encourage you, listen to them (if you like) - don't assume it's just hollow praise - their practiced eye may well be right!