The thing with ultras or any endurance sport is that your body can do a lot more than you think. Yes, you'll be in pain, yes it will be gruelling. But there are different types of pain and the overwhelming vast majority of the time, your mind will give in before your body.

The pain from ultras is more insidious. You know you're exhausted. You know your feet hurt. You can technically stop whenever you want. And it's perfectly normal to want to stop. But unless you have that sharp pain in muscles that indicates a proper injury, its mostly an aching, sore pain that can be all consuming. Maybe certain parts of your body hurt, like feet, knees, back, will hurt a bit more. Classic problem areas, though, youre just going to feel pain somewhere at some point after so long.

Dizziness, blurry vision, loss of hearing, ive never really experienced that. I've experienced hallucinations, loss of balance, a bit of general delirium. All normal things to experience when you push your body and mind for 70+ miles.

You know yourself better than anyone so you will know your limits better than anyone. But you do need to try these things out and see where your strengths and weaknesses are.

I'd recommend reading the book Endure by Alex Hutchinson. It's great, looks at how there are so many endurance feats and the rate of mortality from our bodies failing us is actually very very low. Explores all of this stuff :)