I typically ride for fitness in CP and am never more than a few miles from home (thus I can walk), but whenever I venture outside of the park, I bring a Co2 cartridge, one tire iron and a new tube..sometimes I also swap out the CO2 for a mini-pump as I have had bad luck with CO2 in the past...in the grand scheme of things the extra pound you are carrying is nothing...

takes less than a few minutes to swap in a new tube...much better than hopping on the subway