great info here - thanks everyone for it all!
I'm just starting commuting too and mine is a 6 mile each way, but it's far from flat. the hills are what kills my legs.

but I think i will take the advice of slowing down, especially in the morning - which is doable as there is little traffic.
that's harder for me in the afternoon though because my commute is on a busy road. it's 2 lanes so cars can easily go around me, but because it's a busy road they pile up behind me pretty quick.