Some levels of externalities are sustainable. Some are not. The larger problem with humans is the combination of total population and average per-capita resource consumption (to say nothing of the vastly higher consumption in industrialized nations).

The steel and rubber in a bike are probably the biggest concerns (as well as the paving in roads), but in comparison with an automobile, it's vastly lower, and in comparison with the ranges and cargo capacity of walking, they're hugely higher. I've travelled 200 miles in a day by bike, and commuted over 40 miles daily for over a year at one point. One person on a bike can move a payload of many hundreds of pounds, possibly a ton or more (across level ground).

Food production at present is manifestly unsustainable. Every one calorie of food you eat in the US takes 10 in fossil-fuel energy to produce (in Europe it's about 5). Other inputs, particularly nitrogen (from fossil fuels) and phosphorus (limited global supplies) are crucial limiting factors, as are topsoil and water.

Richard Manning's "The Oil We Eat" is a fascinating exploration of this aspect of human existence: http://www.wesjones.com/oilweeat.htm