You're doing a 5K? Not a bad start.
I've done full marathons--LA (twice), Vegas, and a whole bunch of half-marathons.
My goal is to be ready to do the Durango Double this year, and hope I have the money to fly to Colorado to do it. It's a 25K or 50K trail run on Saturday followed up by a marathon on Sunday. Only problem is, when you're used to running at lower elevations like me, suddenly going up to 7000-7500 feet and doing one can be a bit of a chore.