you are not the slowest ever! i have been 'running' on and off for the last 3 and a half years, and i 'run' about 13 min mile. on my home treadmill i do a 5k in about 40 mins, about 45 mins on the road, and can get it down to 37mins in the gym (super new shiny treadmills that dont require as much effort to run). i always feel inadequate, but then i think "well, its better than sitting on my butt all day"!
i did a 5k race this summer (my second ever) and broke all expectations and ran it in 34 mins, even though i walked a bit (hilly grassy terrain). i was so proud i practically jumped up and down! 
quickly got knocked off my high by a friend who is a new runner and can do 5k in 30mins easily
but then i though, no, i still did well, so forget them!
it doesnt matter how fast others go, i know its disheartening (a friend of mine has never run and is doing a marathon next year, she has just started and can do a 5k no trouble, whereas it took me 4 months to train), but everyone is built differently, and the fact that you're doing something that is hard proves you have both mental and physical strength, so plod on!!!
as for slogans - i love my tshirt i got in freshers week at uni (totally not running related merchandise) that says "i'm busy, you're ugly, have a nice day" 