Really interesting question and the answer is related to this statement:
I'm rarely breathing hard but my legs are burning. and i do not consider myself fit.
Mitochondria live in mostly in slow twitch muscle fibres and create energy by using oxygen. When you do a sustained aerobic activity such as cycling, you typically relying on slow twitch fibres which as great for endurance because they can create a decent amount of sustained energy. As they absorb oxygen from the blood stream, the co2 level rises in the blood stream, this is detected through receptors and this is the driver for you to breath harder.
The nature of fast twich fibres, means they are stronger and can provide more energy faster, but can not provide a sustained energy source (they deplete or cause lactic acid).
Why can you ride for 30 minutes, not breath hard and have burning legs - because you are unfit. Basically, you don't have enough mitochondria and capillaries to supply them o2 in your slow twitch muscles - they can't keep up with energy demands. So you rely on a faster source of energy which creates lactate as a by product (not quite as simple as that), this gives your legs a burning and often hard feeling.
I'll wager 2 things, the ride to work is pretty flat? and you are going just slightly faster than you should (to be relying on aerobic energy system), so you only slightly in an anaerobic state.
Slow down a little, if this is an issue, on longer rides I certainly imagine it would be.
