You might want to buy a one-day MTA FunPass Metrocard, which allows unlimited use of subways and buses for $8.25. (That's worth it if you're taking more than three rides, which are $2.25--though transfers are free.) The FunPass is available at the nifty vending machines inside subway stations, but not from booth clerks.
While they're not as strenuous as Outward Bound, these tours involve stretches of sustained walking. If you have a heart condition, or sore legs, or can't manage a brisk pace, this isn't right for you. Wear walking shoes. It's a good idea to bring water and, in warmer months, a hat and sunblock.
Also, while the main hazard of the tours involve your susceptibility to Brooklyn stoop sales (the urban equivalent of garage sales), some tours (Brownstone Brooklyn and Williamsburg) take us briefly through some downscale areas. Nothing untoward has happened, but you still shouldn't flaunt jewelry or camera equipment. This *is* a city.