Exercise is a problem for me in this hot weather. I get queasy in extreme heat. When I went to a nearby gym, I found myself appalled by the way people smelled (I have a strong sense of smell). So, I force myself to use the stairs at work, don't eat sweets or drink soda, and live (uncomfortably) with the physical results (I have more of a tummy than I used to, and I think my arms are fat). OTOH, how much of that is age? I don't know.
As for healthy food, I do cheat occasionally, but the way I feel afterward or even the next day is motivation enough to mostly stay on the right track.
Much of the process seems to be habit, as in, once you start exercising regularly, it gets easier, but as to how to find that initial motivation, the best thing I've found is to watch a movie about athleticism, such as "Bend it like Beckham" or "Love and Basketball."