You should check out this guy's blog, it gave me a lot of peace of mind after reading--
http://ecumenicity.blogspot.com/2007/10/navy-ocs-tips.html
I know what you mean- after my stress fracture, my biggest worry is just being able to do the 3-4 mile run at the 9 min pace. Every little pain I get in my foot makes me question my body's ability to recover, even 6 months later and though the doctor told me the fracture was caused by my shoes.
People tend to over-state the running that occurs there. You start out the program doing 1.5 mile runs 3x/week. There is a warmup formation run (very slowly paced) and a short warmup run, all told totaling 1.75-2 miles (so somewhere around 6 miles on the week). You are also broken up by ability so if you are in the slow group the staff leading the run will run at about a 8-10 min/mile pace, depending on what the group can hack and how the staff member is feeling that day. You may also work sprints in there to get your speed up. After about a month, the distance gets raised incrementally. We did 2.2 mile road runs but now people are saying you use the track. Tues/Thurs are strength and conditioning days where you do all sorts of creative stuff that will make you wish you were running. I was not a runner showing up to OCS but after 2 weeks of these days I looked forward to the run days. They were relaxing. No one yelled at you unless you were last, so don't be last (You won't get picked up in the van unless you demonstrate that you can't continue running. Don't do that unless you are injured). Crawling across a field in a caterpillar formation while the DI yells in your ears and doing it again because you didn't beat the other team was not relaxing.
Thankfully strength and conditioning gave way to suporting graduation preps for the graduating class. We had a small class so between that and watch PT on tues/thurs stopped occurring.
Sat is a pot luck day and it's pretty much going to depend on your DI's preference on what you do those days. Your first Sat is outpost when you move from the indoc p-way to your batallion p-way IIRC so you can bank on that one involving a lot of creative exercises with a full duffel bag. I did 1 5k in the 12 weeks I was there, and that was the longest run. One of the more fun ones was going to the gym and having races with another class (when I say race I mean crab walk, bear crawl, etc and loser gets to go again while the other class yells at them for being losers).
If I had to emphasize one exercise that would make life easier at OCS it would be pushups, not running. For my class it was bear crawling, for another it was 8 count body builders, and for the black ninja's class they just spent a lot of time in the down pushup position and 'making diamonds.' But the common theme is that regardless of the DI's flavor you spend a lot of time on all fours and being good at pushups will make that time a bit less painful.
So bottom line: don't show up a fat slob, but you don't need to be a world class runner to be capable of running 6 miles a week at a 9 min/mi pace.