5Ks and beyond

We decided we're going to train for the Disneyland half marathon in September.
Since we're both overweight and lazy, this presents a unique challenge for our bodies and our minds.
But we're going to start off slowly. We did several 5Ks last year, starting with the 5K that was part of the LA Marathon. Our time there was 1:06, and we've done under an hour each 5K since. My personal best is something like 48-50 minutes for a 5K in Malibu last year.
But this half marathon seems fairly daunting. It will be the longest event either of us has done in both time and distance.
I can't imagine spending three and a half hours doing something I like, let alone three and a half hours running (the half marathon has a time limit of 3.5 hours, meaning we'd have to maintain a speed of something like 3.75 miles an hour to make it. Right now, we're at about 3 miles an hour.).
Luckily, there are plenty of training programs available online.
The bad part? They all assume that people seeking these plans have at least a passing relationship with exercise.
Exercise and I are like bad one-night stand participants: I get all dressed up, we meet and flirt, and have fun for about an hour, but afterward, I stumble out into the morning light, muscles crying and head pounding.
But, for now, my next step is running a 5K. I think I can do it, at least, if I can manage to keep myself from stopping and taking pictures every quarter of a mile.
Dave's thinking big, though: A couple days ago, as we were talking about the half marathon, he asked, "So, after we complete this half marathon, do we want to think about running a marathon?"
It's good to dream.