There are almost certainly more runners on the towpath than anyone else. On every trip to the towpath, TowpathGuy sees lone runners, pairs of runners, runners with dogs, ROTC runners from Rutgers, etc. Some of the runners are slow and moving along quite slowly; others are obviously quite good.
Where to Run?
Running on the towpath is easy. Use the Towpath Access Guide with Maps to find an entrance near you, put on your running shoes, and go. The maps can help you figure out the distance you want and given the length of the towpath even ultra-long distance runners can get a good workout. Another way to find a run on the towpath is to enter the name of the entrance that you want to use, the word "towpath" and "mapmyrun.com" into your favorite search engine. There are a lot of mapped runs that use the towpath.
The towpath surface is hardpacked throughout its length. About the only thing that a runner can't do on the towpath is hill work! It's really flat.
Some towpath runners carry water bottles (sometimes several) strapped to their waists or wear water packs on their backs. Others seem to leave water containers at entrances along their routes. Towpathguy has seen group organizers with large coolers and cups waiting for the runners to pass by.
Organized Runs on the Towpath
The Raritan Valley Road Runners Club -- "New Jersey Best Running Club" -- has a group run on the Towpath every Saturday at 8:00 am, leaving from Johnson Park, near Landing Lane and going "from 4.8 to 20 miles" to "forever", if you want(!) but the idea is that this is a low pressure support group. Runners of all levels are welcome.
Running Races on the Towpath
There is one organized running race on the towpath -- The Fred Almgren Memorial "Mayday" Race. Every Spring, early in May, since 1976(!) the Princeton University and Rutgers University Math Departments have a relay race. In even years the race is northbound from Washington Road in Princeton to New Brunswick; in odd years the race is southbound from Landing Lane to Alexander Road. Last year (Spring, 2008) there were 11 teams competing. The history and stories behind this relay race are fascinating and worth a read -- visit the website!
