This is no problem!! I have done it in Florida many times.
Basically, just go into any restroom with a sink--preferably one with the sink in a private stall (handicapped stalls work well for this situation--or some office buildings have one room restrooms where you lock the door and the sink is included in the locked area--this is what you need).
Take off sweaty running clothes. Turn on water. Splash water all over your body, everywhere (yes you will get floor wet, but who cares? They are made of tile this isn't a big deal). Now put soap in water, lather up your hands, and wash everywhere, quickly (maybe not as thorough as if at home but as well as you want--you will get the hang of how much you want/need). Now comes the messiest part--again splash water all over your body from the sink (cupping water in your hands obviously). More water will be getting on the floor, but again who cares. Get all soap off. You have just showered.
Now, obviously, dry yourself off. We are used to sweating in Fl after runs--this isn't a big deal you will feel clean just like you showered at home after running in Fl and then got dressed at home.
If you are in relatively clean office (esp. if this is where you work), take towel from bag and dry floor. Or bring old second towel for this purpose. I've also left the floor wet at places like outdoor parks and/or McDonald's--they usually have a drain in the floor and I figure the water will dry and they will clean it (the nasty factor makes me not want to use towel in these situations).
I've done this at away races, again if outdoor nasty restroom then I don't clean floor. And I've done at an office where I worked--at which time I did clean the floor esp b/c co-workers would come and see wet mess.
The key is that you have to find a bathroom with the sink inside the locked area. This might take a little looking but they are around pretty much everywhere.
The hardest part for women is our hair--not very easy to do your hair in this situation and make it look decent. I would just run my fingers with water through my hair a little--then put back with a clip for the afternoon. For men, this should be much less of an issue (which is why you guys have it so much easier!!)
I always felt great after these showers--honestly just like a real shower!!