I saw your initial question and straight away thought of what others have already said...
I'm 27 and have also noticed that I've been getting bigger and bigger over the past few years. Before that, I would yo-yo, which also isn't healthy. I'm now seriously trying to get myself fitter and healthier as it really hit me that if I didn't there is a very high possibility that I am going to have problems later on in life.
I've started using the My Fitness Pal app and website daily and track everything. This is allowing me to see what I've been eating. Some days I'd be really over my calorie allowance, others I was really under. I always knew what foods are good/bad, but logging everything and actually seeing the numbers on screen is giving me more motivation. For example, one day last month I was really under on my calorie count, but almost all the calories I had had were from sugary drinks. I was under, but in no way healthy. I had been so busy at work that day I hadn't really eaten, just drank coke to get through the day. Of course it's something I already know - a diet of coke is not the way forward. But like I say, seeing it on screen can be a big shock and kick you in to eating better. Now I'm beginning to ttc it's even more important I stay on track and am eating healthy stuff, and mfp really does help in that sense.
Walking - a great exercise if you've not really done much exercise in a while. That and swimming. I love going swimming. I control how hard I feel I can push myself each time I go, and even a slow gentle session involves pushing through the water. I've also started going to an aqua zumba class. I feel more confident doing things like that than going to the actual gym as you're mostly underwater so I feel like people aren't watching me.
Feel free to add me on MFP if you want. My user name there is the same as on here.