I agree with the challenging bit, but for a different reason. Quoting from Piers Steel, "We are motivational misers who constantly fine-tune our effort levels so that we strive just enough for success."

For low complexity goals, up to a point, there is a linear relationship between goal difficulty and goal performance, even when the reward is held constant. That is, more difficult goals require more motivation; provided that the goal is valuable, that motivation is provided.

The difficulty with choosing challenging goals is ensuring that you feel motivated, not excited. Excited is thinking of the benefits and feeling positive emotion in anticipation. Motivation is the energy necessary to actually complete all the crap in between, like doing your pushups. I use mental contrasting for this. It works better than nothing, but still leaves much to be desired.