My commute is between 17.5 and 20 miles each way depending on choice of route, the shorter route has more uphill on the way in though.
If I feel weary with it I have an 'easy' day and say to myself that I'm not to change gear and choose small ring (42) I think and say a (19) sprocket once I get into it I usually end up changing up a couple of times.
I have a Topeak rack and bag and a bikebins.com pannier box, I'm happy to ride loaded up because when I get on the race bike it feels super agile etc.
In the winter I have a couple of cateye battery lights on the front and carry spare batts, on the rear a selection of leds, up to three between the pannier and my helmet.
Clothing wise I cant do waterproof unless its peeing down, just get too hot. Layers are merino thin and medium and a l/s club jersey over the top, never cold last winter. On the legs I wear dhb merston bib tights, very comfy and at £37 great value, waterproof overshoes are a must. For gloves I manage with windstopper fleece ones, never needed anything more.
I dont ride if it looks frosty, too risky.