Yes, I have a couple suggestions.

The first suggestion is to try to start thinking of yourself as a fitness buff and actually do things to make yourself start thinking this way. So for example when you are waiting for the morning train, you could glance at fitness magazines instead of what you usually look at. Or you could occasionally buy an inexpensive fitness-related chochke, for example a wrist gripper to keep at your desk at work. Make an effort to look at other peoples' bodies and note who appears more fit.

So on a cold dark morning when you are deciding whether or not to go the gym, the reasoning should be "I am a fitness buff and fitness buffs go to the gym every day" as opposed to "I want to be stronger/fitter so I should go to the gym."

The second suggestion is not exactly a goal substitution but it's a pretty similar concept -- what I would call a tie-in. Tape a blank piece of paper to the wall of your office or bedroom. After each workout, draw another tick mark on the paper. For nerdy people, there seems to be a lot of satisfaction in accumulating points. So you are basically tying the goal of getting strong/fit to the goal of accumulating points.