Distance or Speed first?

[TsPoet]

I'm overweight and always hated running. I went to Vibrams (see MDHillSlug's thread) and can't help myself, I've started running and I feel great! I run off-road so I can walk my dogs at the same time.
I've also been asked to be a mentor for this organization
http://www.girlsontherun.org/
which coaches young girls that they can do whatever they set their minds to and attempts to empower them in general. The culmination of a 12 week training period for these girls is a 5K run.
I can run a mile, then I fast walk my dogs another 1-2 miles. So, if I can run the whole walk, I can do the 5K with the girls.
Oh, I also jog slow (17.5 min miles!). So, what do I do first - speed or distance? I ran a 15 min mile last night and it felt great. But, is it better to slow back down and go for 1.5 miles? I don't care if I average 17.5 min miles for the 5K (mixed units, I know), but the faster jog last night felt good, so maybe I can try for 15 min miles for the 5 k.
My mentoring part for these girls is not in the physical fitness arena, so this is extra, but I like the goal for myself. Any advice on how to build up to a 5K (in 10 weeks now), would be appreciated.
Thanks,