I have been riding geared bike and road bikes all my life and today i built my first fix gear , it is a 1980's schwinn sprint i converted to fix with a wienmen rm19 rear rim on an origin8 flip flop hub running a 52x18 gear ration. I love riding the bike but i have a couple question. First how long will it take me not to fell like an out of shape slob when i ride more than a mile on my fix bike(the ratio fells fine i thinks its the constant pedaling) and second what is an effective way to slow down fast, i kept the fornt break but im accustom to being able to slam on my road bikes break and stand on the pedals and stop near instantly.

Thank you

Chiken

Here is a picture of the bike as i got it , will post pictures of it in fix configuration tomorrow when i take them.

Your front brake is the fastest way to stop, just keep your ass on the saddle so you don't flip over (geared bike and all others)

Lower your ratio a little, like 52/19. Even if you think it feels fine, starting from a stop can wear you down at higher ratios. To slow down fast, backpedal + brake, or maybe put two brakes on.

Not sure why you are tired? 52/18 is not a bad choice to start at, or do you have a lot of hills? If you want to stop faster, why can't you add a rear brake?