I'm a long time bike commuter (over 10 years), I was living some 4.5 kms from my workplace, a mostly flat commute (well, maybe 40 m up-and-down). After two years of that commute, it had made my significantly healthier and fast.
A year ago, I moved to a place a bit closer to my work, but much higher. I have now a 3.3 km ride home going 160 m up, most of which in the last kilometer (maybe 10% average in the last bit). While I had no problems with my commute just after moving in, over the time, it became increasingly more difficult, and now, I always have "heavy" legs when I start the ride home, and I take much less pleasure in my commute...
Do you have tips to avoid this ?