15 is a nice sweet spot for commuting ... I'd definitely encourage you to do it!
I've been commuting 24 miles round trip this year (summer/fall). I'm a fair weather commuter, and for the last week I've been fighting off a head cold so I haven't ridden, but I was hoping to today and just didn't want to push it.
Back to Back days are tough ... especially if you're not used to them. That said, you can vary the intensity too. A lot of times I'll ride hard in the morning and then leisurely on my way home.
No need to change your bike ... maybe some slicks as has been suggested, but that's about it
