As far as I can there are like three REALLY portable endurance sports: swimming, running, and weight lifting. “Portable” means you can work-out almost anywhere without much gear. There are some great online tools to make your workout less logistically taxing while you’re adventuring far from afield:
Some people can just go out and run, and some people need to plan a route and have tangible goal associated with their daily fitness adventure. For those who like to plan their route there is Google Pedometer where you can route plan your next road or trail run. If you are interested in a topographical profile view of your run check out this handy plugin to google maps, Path Profiler.
Find a local pool on Clubswim.com. If you prefer open water swimming it is not hard to find a hole and query local water quality. My three favorite open water swimming holes are Philadelphia River, Atlantic City Oceanfront and San Francisco Bay. It’s very likely that whatever city you visit (in summer) has open water swimming and that a google search query for the city name and “water quality” or “water department” will find you the site you need to feel safe in the water.
If you’re traveling and need a place to pump some iron check out this online resource (Gymticket.com) of 20,000 gyms.
I’ll leave you with a quote from fitness guru Charles Atlas.
“15 minutes a day!
Give me just this
and I’ll prove I can
make you a new man.”