I actually didn't change surfaces, I've always had what I consider to be a pretty good mix of soft surfaces to keep from beating the legs up and hard surfaces to prepare myself for what I'll face on race day. As for what you're doing, maybe going all the way to flats isn't for you. In my opinion, it's more about not wearing more shoe than necessary. If the lightweight trainers are what's necessary, so be it. For me, the flats seem to cut it for at least everything I've done since making the switch so far.