As to being born with extra willpower, that's bull.

http://www.springerlink.com/content/n3846778w553317l/
51% heritability of delay discounting at age 14 in a twin study.

Your case is that a pill which mimics the physiological effects of exercise would fail to mimic the mental benefits of exercise, because humans require adversity to build willpower. Widespread use of the drug would have the negative side effect of making people lazier, but would still be overwhelmingly worthwhile to universally prescribe it for the medical benefits. Is that a fair rephrasing?

I had missed the "still worth it" part from your original post, my bad. The McDonalds idea was a poor choice of example; I think when you accepted the reversal proposed above it was not really reflective of how you actually behave. If you accept that "it's okay for the government to make my life harder for my own good" then I would like some examples of how this belief has shaped your behavior. If you want to change your mind and not agree with that reversal after all, I would not fault you;

I can think of three possible ways to eliminate the downside of such a pill.

The first is that much as today we educate people that exercise provides important physical health benefits, we instead educate people that exercise has important mental health benefits. So, even though they're already in great shape because they're taking this pill, they still need to go lift really heavy weights that are still challenging so they build up willpower.

The second is that we could provide people with some alternate form of adversity to replace exercise. Maybe we could make the education system much more challenging or put everyone through military boot camp. I'm sure we can figure out some way to make people's lives harder if we wanted to, and I bet universal boot camp would really toughen people up.

The third is to come up with a drug that mimics the mental benefits of exercise/adversity.

The first solution has the problem that it would work even less well than telling people they need to exercise does now. Most people wouldn't bother, just like most people don't bother to exercise now.

The second solution has the problem of deciding what the optimal level of adversity is. If people are allowed to choose their own level of adversity, I think most people would choose not to make their lives any harder than they need to be. If the government is setting some minimal level of adversity, what level do you pick? How do you know that the optimal level wasn't the level of a poor peasant farmer in the 1700s (bet they got a TON of exercise and had boatloads of willpower)?

The third solution has the problem that you think it is impossible. I disagree; I know of no laws of physics which disallow such a drug. It might be extremely difficult to create, but if there is an arrangement of atoms in your head corresponding to a brain with lots of willpower, there is nothing barring another brain from being modified to have the same arrangement of atoms. I suspect that the vast majority of the increase in willpower is due to a modification of whatever part of your brain implements willpower, and that the influence of knowing you worked hard for that willpower is comparatively minor, but the point is moot. If we have to design a pill which gives you false memories of working out for eight hours a day for the last twenty years in order to duplicate the effects of willpower, no law of the universe prevents us from doing it. If instead of a pill we have to destructively scan your entire brain after freezing it and grow an entirely new copy of you from scratch, the point stands. Willpower comes from your brain, brains are made of atoms, atoms do not have memory per quantum mechanics, therefore willpower cannot actually require exercise. The fact that it currently requires exercise is merely an unfortunate fact of human brain design.

If you want to argue that it would be less fun or elegant or beautiful to use a drug to gain willpower, that is an entirely different argument than whether or not it is possible.

Do you have any background reading that you think would illustrate your reasoning better? If we still disagree, we must be reasoning from different background information, and if there are particular sources that influenced your reasoning I might take a look at them.

If you're interested, my own reasoning on the topic draws on evolutionary psychology (especially The Blank Slate by Pinker), reductionism, and the transhumanist analysis of the human mental tendency towards 'sour grapes' (Yudkowsky has a write up of this somewhere that I can dig up and link if desired).