2. Sustainability is what matters.
Does this activity add to the likelihood that humanity will cease to enjoy a standard of living similar to the current average at some point before other natural processes (Sun going red giant, etc.) kill us all anyway?
So hey, if they sun's gonna kill us all next year, go hog wild. If what I'm doing now means that somebody 500 years from now will have a more impoverished existence, it's not.
Advanced species lifespan seems to range from ~1 - 10 million years or so, plus or minus a lot, before genetic drift sets in.
Three hundred mega-years (what I meant) is a somewhat arbitrary milestone. (I think long-term but 10^15 years is a bit much for even me. But see Stapledon's "Star Maker"...)
(BTW, your thinking seems unusually clear on these matters.)
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