Good aerobic run

Went out for a 12-mile run today. Was trying to keep my heart rate around or below 80 percent while going faster than a crawl. Results: Did it!
Ran 12.04 miles in 1:49:05, which is a pace of 9:03 minute/mile. Didnt drink any water or anything during the run. Just had a banana about an hour before the run. Heart rate average was about 79 percent with a high of 83 percent. That's what you call running aerobically. Anybody can pour it on and run hard and do a 9 minute mile, but to be in shape aerobically, its important that you have the ability to run fast without having to use too many of your fast twitch muscle.
This run puts me at 42 miles for the week. A little high, I agree, but, I want to be in great shape before I start marathon training in August for the Dallas White Rock Marathon so when I have to run high miles and log 3 hour runs, it won't hurt as bad.