First brisket ever and I don't know what went amiss. Marinade? Coke a Cola. 24 hours. (I thought it sounded funny but FUN) Followed epicurous recipe but i didn't have roaster sealed tightly. Dried up BAD. On the bright side my dog will love it as I can't serve this to anybody with good conscience! Was it marinade too long, or covering? Cooked at 325 F for 3.5 hours.
Almost certainly it was the inadequate covering (the roaster not being sealed tightly).
Marinating for too long is more likely to produce overly-tender - that is, mushy - end results (whether 24 hours is too long for a soda-based marinade is another question, I think, but I've had Coke-marinated meats that have been great).
Brisket needs moist heat - slow cooker, lidded roaster, or tightly covered with foil. The temp you used seems reasonable. The time isn't overly long, but the specific time needed depends on weight and frequently is stated along the lines of "for three and a half to four hours, until the brisket is fork tender."