Sure we all want to be healthy, slim, and live long. The problem is not motivation (well for most of us). The biggest challenge in my opinion is establishing and following a routine. I found my routine by luck, but it takes discipline to follow any routine.
The passion of music and being able to sing was all the motivation I needed to go for a swim right before singing practices. I would try to practice singing everyday, so I went swimming everyday. It didn't matter what time it was time when I got back from University, and how much work I had. Plan was set: swim first, sing after, and school work later. Yes my grades did suffer a bit, but my life improved drastically. Within 2-3 months, I found myself going swimming just for the sake of swimming. It just felt so good after. If I missed a day or two I would start to get anxious easily again. It was almost as if I was hooked on something: Endorphins! Soon it wasn't just swimming anymore. I discovered running is even more intense, more endorphins in shorter time. I know at this point I sound like a drug addict, but this drug is free and seems to improve every aspect of my life: physically and mentally ...so why not!
Now this is all nice and easy for a college boy who doesn't have much to worry about beside school and student loans.... yes again I'm lucky in that sense, but it did require serious discipline. Coming back from University after 12 hours of lectures/labs, getting dropped off at a bus stop with 15 minutes of walk under the rain, doesn't exactly make you want to go swim or run does it? There is a moment when you know, something is rewarding after you do it, but the easy option is also there instantly (the couch and the warmth), that's when I needed to push myself - its matter of keeping things moving.... its simple physics takes more to restart the movement than just keep it going. You skip one day, next week might be 2-3 days.
I am now a 31 year old professional geologist, and not as free as I use to be in college. I admit, I don't know what's going to happen once I have a family with kids running and pooping all over, but I look around and sticking to a fitness routine is not something I see often, and I am very proud of keeping the ball rolling. I treat exercise (running, swimming, biking, or some gym work outs) like brushing my teeth. I would feel horrible for missing a day. Even if I am in the field sitting in a truck or a helicopter for 10 hours (not all field work is hiking in mountains), I will run a cross a remote camp in a middle of a swamp if I have to, just to do my minimum daily 25-30 min of exercise, in fact, I've done that! At home it's easy, even if I had a busy week and feel like I haven't been exercising as much as I should, I will gladly sacrifice Friday night beers with friends or co-workers for a nice bike ride or a run. From my experience it is sometimes hard to get started, but I have NEVER been disappointed after finishing an exercise - in fact in makes a seemingly unproductive day feel productive.
Here is what it comes down to, lets say It was a long day yesterday with an after work event and it'be been a long day today: is it going to be couch time and re-run of Seinfeld? Reading a book? OR 25-30 min of elliptical in small windowless gym... maybe a run outside in rainy weather?... I feel like that's the test for fitness routine keepers!