mike, I can put the thinnest/slickest tyres I can find on my 6" full susser, but it is still going to be more pleasurable commuting on a bike designed for commuting. The mmmBop is not quite such an extreme example, but the point remains the same.

A bit of hard work will make you fitter 😉

you don't sell road bikes, do you? 😉

miketually – aren't bikes made for riding/ruining?

Absolutely, but I prefer to ruin my MTB with mud and smiles. My commuter gets ruined by salt and rain.

A bit of hard work will make you fitter

My commuter's twice as heavy as my MTB 🙂

Two 150 litre bags of compost on Yuba Mundo by miketually, on Flickr

More, when it's got 300 litres of compost on the back.

No, I don't sell road bikes. I don't sell mmmbops either, but I suggested the OP get it anyway.