you'll find plenty to do on the Elan valleys for long runs I have some excellent routes up there. Regards water, I've been drinking it there for 15 years no worries. But, this time of year be sensible especially with the low water levels and low rainfall, don't drink "stale" water. Look for something running a bit and obviously better on high ground. You will hear some underground gutters running, if you hear one and don't fall down it and break a leg - have a drink! I've never been too worried about dead sheep, but I wouldn't drink below a farm.
Can PM me if you want some routes, I know where the paths are etc.