Can I marinate for 8 hrs? Will this ruin it?

Agree with amysarah and HalfPint -- 8 hours should be totally fine!

8 hrs should be ok. Beef is dense and there's no acid or enzyme in the recipe that would degrade the meat.

I've set this to marinade in the morning, for grilling at dinner time - so 8 hrs+. It's fine - if anything, the flavor permeates the meat even better. (I do this with most marinades for beef; unlike, e.g., fish, it holds up to a long bath.)