Aside from the f stop and high ISO, you're also about to take a crash course in AF tracking! The D50 doesn't have blazing fast AF module, and the 70-300 is no spark either. But, if you have enough light, you should be able to get the hang of it with AF-C mode, keeping an eye on which AF points are locking.

Or, if you have a repetitive performance circuit with the dogs, you'll want to lock focus in manual on an area where you know the dog will be passing through, and depress the shutter when it gets there. No flash...bummer.