Short daily commute (8km), preparing for 100km race
 

My commute is twice 4.2 km, on flat road, in an urban area. Last summer, I went for a longer ride with a friend, and I was surprised to be able to do 100km at a good speed. I was exhausted, and I slept well.

I have continued my 8.4km daily commute, even throughout winter. Every week I'm going downtown and do a little more distance (10 km more), and my legs feel the extra training. I'm pushing it just a little bit more.

In exaclty two months, I'm participating in a 100km race on hilly terrain. I have started doing an extra 20km training every week, and I find my legs are doing very well, even at climbing. My breathing, however, is what's keeping me at lower speed.

What should I do to get in shape for a 100km race? Does it matter if I can't push too much in climbing, knowing that my main asset might be endurance?