Should've run!

Did you read the FAQ? You can't run, it puts more stress on your heart rate. It's better to try to stay calm-headed.

got blunt?

I was screaming at my tv....telling him to run..run..run....

I agree!! He should've opened the theater door and got a running start. He thought about it way too much. Should've run and looked just at her on the other side.
I'm not aware of any FAQ saying they couldn't run. Anyway the movie never made that point so that's what I based my opinion on, not something that you read after :)

I think the idea is that folk caught in the open die from psychological panic, not some environmental toxin or pathogen. The panic begins as they approach doors or even windows, and grows to near unbearable as they think of stepping outside. So they’d likely be unable to force themselves to run out.

Some people succumb to the panic sooner and worse than others, so there’s not set time before “damage becomes critical”. Though an act of will, Marc is able to hold it off almost long enough to cross the street – had the distance been a little shorter, he would have made the walk without falling.

My thought was that he should have tied a sack over his head, making him unaware of when he was inside or outside. They established earlier in the movie that being inside a car doesn’t prevent the panic, so just holding an umbrella over his head wouldn’t have worked, but perhaps a scheme where he didn’t know when he was outside rather than inside, so didn’t know when to panic would have.

Similarly, If I’d been Mark, I would have taught my child to pull a cart with an lighttight box on it, allowing him to cart me and Julia around, and as soon as we could, teach him to drive and acquire a car.

However... Marc was taken completely by surprise originally when he tried to get home to his girlfriend. He was knocked off his feet by the effect of being outside. He actually seemed much more stable when he knew what was coming. He rather reminded me of having food poisoning. Everything seems easy and straightforward and "I'll just get up and go to the bathroom" and before you know it the world is spinning and you're on the floor before being helped back into bed. So, I think running wouldn't work, you'd just end up in the wrong place faster and land harder.