I'm not coming from big beefy, but most years I let the road bike sit a while in the winter and only ride the MTB-style commuter. The first road bike rides each season can feel weird...

My best advice is to go easy, give yourself time and/or space (I often head to the path first before doing some of the more heavily trafficked rides) and definitely try little skills drills. For example I will use sticks, big leafs, etc. as an excuse to create an imaginary obstacle course to get used to the different handling. I'll also ride a particularly hilly short course early in the day before the cars take over, and do it a bunch of days in a row, just to get the hang of descending in the drops smoothly and confidently. Climbing, well, that always seems to come naturally.