Slow Cooker: Recipe says Medium, mine has high and low only
If it's a wet recipe (casserole, pot roast) you're more likely to find the meat falling apart than tough. 

Medium is non standard and there's actually quite a bit of variation between slow cookers. Medium on our current model is pretty much the same as high on our old one; high on the new one is a bit fierce for most recipes without adaptation. 

I suggest that you cook it on high, but if you're around when it reaches the minimum cooking time, then turn it down to low (not warm) until you're ready for it.