I poured an epoxy river cast and it took off on me even when in a cool environment and with fans.
I'm looking at fixes for this without chipping everything away.
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As I'm sure you'll have picked up from elsewhere, you can't repair this. I will bet you'll find at least someone who's tried to recover from something similar to this by pouring in more epoxy but clearly (pardon the pun) the best idea here is starting again. Even assuming you patched the cracks and somehow they turned out well enough to be acceptable you have obvious fracture planes in a few places at the wood/epoxy juncture, and there's no coming back from those. In addition to how visible those fractures are, they may actually indicate that there's no bond to the wood at all.
Just in case it is an idea that you considered, it's not viable to refrigerate one or both of the components before mixing. Cold-weather recommendations for epoxies almost always call for the liquids to be pre-warmed before mixing, both to help with accurate measurement and also to ensure proper curing.
If you want to continue to use what you have it's worth investigating if there's a slower hardener available as that's one of the classic ways to control temperature rise in resin mixtures.
in a cool environment and with fans
It's possibly worth pointing out that fans don't cool air.
They make us feel cooler because of evaporation (that's what sweat is for), but air itself is a poor conductor — it doesn't carry heat away efficiently — and there's zero evaporative cooling with epoxy since there's no water in it.