Remember, right now you're not based on speed - but time. It is, really, the "secret" of c25k, ohr, and others.. which is for you to figure out WHAT your pace should be to sustain it for the duration.
Once I broke 5k, 1 hour, 2 hour, 3 hour - these came pretty quickly.
I finished c25k on 4/3/05. I did 1/2 Marathon 3/06... 5/06... 8/06 (1/2 Ironman), and so on. Am I fast? Nope. But I can go the distance and each race, I am faster...
so you can do it, just hang in there, but remember the REAL challenge here is controlling your speed. It's harder than one might think!
(p.e. 5'9", 235 down from 322lbs 1/04)