45 minutes is a long time.
Also, you only have 8 GB of RAM? That's a pretty small amount for AE work.
Try rendering to an image sequence (don't worry, you can put the picture back together with the audio later in Premiere).
Then, when/if it stops working, that's okay. Simply start another render, but only render from the frame it stopped on. Is it able to start the new render reasonably well? If so, just do this until you have every single frame you're meant to. Import that image sequence into Premiere and put the audio with it. You'll find Premiere does a much better job of making an h.264 that AE did anyway.
It's probably that your particles were never dying and you're running out of RAM. Make sure you set a reasonable lifetime for your particles.
I just checked the particle settings. Life is set to 10 seconds. Life random 100%. They fade in, float around a bit, then fade out.
RAM never seems to exceed 70% while this is rendering. I have AE allocated to 7 of my 8GB.
Good idea. I thought of this also but wasn't sure the best way to execute it.
What render settings would I use to accomplish that then?
Unfortunately, can't upgrade the RAM on these new iMacs