Old thread that seems to be revived by one of the original subjects, but I'll answer anyways because I know other 30-somethings will read this. Short answer: yes, totally possible.
I'm 30, about to turn 31. Ran a 16:15 and 9:42 3200m in high school. I just got back into it after not training regularly for 10 years. At most, I would train lightly for a few weeks consecutively for a 5k or 10k once or twice a year, and then fall off until the next year. Rinse, repeat.
This year, I started trained for the annual 10k, and something weird happened: I enjoyed training again. Maybe it was the nice weather, maybe it was that that I was running on pure hate after some relationship troubles. On about 6 weeks training, I ran the 10k right under 6 min/mile, which I thought was possible but wasn't sure.
Decided to test my fitness some more and jump in an upcoming local half marathon, just to see what I could do at that distance (also never ran a half marathon). Was looking for a NYC marathon qualifying time (1:21 half marathon), but ended up cruising along and ran right under 1:18 (5:57 pace). All that on about three months of consistent training after a decade of not training consistently.
Since then, I've just tried to maintain and build upon that. Jumped in a 5-miler over the week and ran about 28:00. Hoping to jump in a 5k soon and run under 17:00 for the first time since probably 2005 (just ran a 2-mile tempo in under 5:30 pace, so i think it's doable).
Anyways. Get a couple months of consistent training with some long stuff, hard stuff, hills, and easy stuff. Who knows what could happen.