Getting out there and doing your run, or your routine in the gym, is the 80%. Other specifics will make the remaining 20%.
Both of those turned out to be huge fallacies. I'm 40 now and not very serious about working out, but I do it reliably (on an irregular schedule) and I don't target specific muscles etc to try to make myself look a certain way. The result is that I am just plain healthier and feel great. That has many second order effects.