Well, according to common sense, that would make sense. Granted, I think Penis Enlargement will still work regardless, just perhaps not as well.
For instance, if a person smokes, then body capillaries shrink and get harder. This in and of itself would limit a person in terms of erection quality and therefore would reduce any positive effects from jelqing or other erect exercises. However, if this person did solely stretching exercises, the fact that he smokes wouldn't mean much of anything.
So, I think if you are in better physical condition, you will probably do better. However, this extent in a lot of cases might not have a significant bearing.
Now for a quick little note on my own situation:
Over the past few months, I have been making sure to run, lift, and do Penis Enlargement on a regular basis. Doing all of these together is improving my physical condition in a synergistic way. Also it is improving self-confidence, which I think is very important.
So the way I look at it, if you can, improve your health in all aspects, and not just Penis Enlargement. In the end, what's the the use of having a big dick if you don't have a body that is equally as well equipped to use it?