I don't know about any aerobic enzymes that would really change that much over an increase in mileage from 60 to 80mpw.
However, like the rest of the letsrun crowd I like higher mileage....the higher the better (as long as you don't get hurt). So...yeah, do 80mpw. I'd personally do over 100mpw for an 8k.
The increase in mileage will make you stronger, more economical. Long runs over 90min are debated to help stimulate your fast-twitch muscle fibers and may even help your speed.
And your aerobic processes will become more efficient...read: higher mileage= adaptations in the circulatory system (more efficient blood transport) increases in the size and density of mitochondria (sp?) in the cells and coupled with adequate intensity in training your lactate threshold and V02max improve....which BOTH are going to help your 8k.
I mean an 8k is almost a 10k. Thats a pretty long race...you want to be as strong as possible- you are better off running 80mpw than 60mpw bottom line (unless you are injury prone).
my running website:
http://www.geocities.com/sagecanaday/