My commute is pretty similar at 14.1 miles. and I try to do it at least twice a week. The big difference from my rubbish miles last year to much better performance this year is the clothes. I have got a winter bike (Cyclocross), and although this hasn't been the difference to me, at your distance I can see the advantage of a lighter drop bar bike.
I terms of clothes, compared with last year.
1) Better gloves (Only Aldi winter gloves, but meraklon liners)
2) neoprene overshoes
3) A skull cap
4) Base layer plus autumn jersey plus windproof shell. Having said that I am getting a fairly expensive Castelli winter jacket this week
The right clothes are crucial, it has made a massive difference to me, but a winter bike is nice. I wouldn't fancy 14 miles on an MTB.
Dave