I'm 49 and have been exercising (hard!) on a regular basis for the past 11 months and I feel fantastic, both mentally and physically!!
I was very fit in my early-mid 30s then work pressures / life took its toll and I fell off the exercise/healthy eating wagon. Fast forward to a May last year and I was 30 kg heavier and got out of breath climbing stairs.
Finally I decided enough was enough and started seeing a personal trainer 3 times a week and eating clean. It was HELL at first. I felt ancient and useless. My body wouldn't do what I wanted, and I was embarrassed about the state I'd let myself get into. I knew going in that the mental battle would be the hardest thing. I hadn't set foot in a gym for years and when I looked in the mirror, it was no longer the fit body of a 30 something. It really brought me back to my very unpleasant reality as an overweight, unfit middle aged woman. My trainer knew me from years ago so he knew what I was capable of (despite appearances!) and he worked me hard. Without the first minutes of my very first session he had me doing deadlifts. Almost a year later, he still works me hard but obviously my workout is a 100 times tougher than it was when I started. I remember when I was fat and trying to motivate myself reading the quote "One day your workout today will become your warm-up". So true!
These days I do weight lifting/strongman work with my trainer 3 times a week, plus I also do HIIT / circuit training classes 6 days a week. And I walk everywhere. My body has totally changed. I thought that flabby, saggy skin on my bingo wings was me getting old and there was nothing I could do apart from accept it. I was so wrong.
But more importantly, I feel so strong and I have so much more energy. I literally have a spring in my step all the time. I'm the oldest woman in my gym. I used to be the fattest - and oldest - woman there! Now I'm stronger and fitter than women (and in some cases, men!) half my age. It feels fantastic and it gives me so much more confidence. I've spent years hiding away in black frumpy clothes, taking the escalator rather than the stairs and avoiding beach holidays and dreading the summer when I can't hide under quite as many layers of clothes. People are genuinely surprised that I'm 49.
It took me over a decade to get my act together and get myself back into good shape. And despite how good I feel now, it's so easy to fall off the bandwagon (I ate a big bag of Caramac buttons earlier today) so I try to find a balance of treats vs eating healthily. I added more gym classes to my schedule to balance out the extra eating I'm enjoying!
For me at 49 all the hard work has been worth it. Every day I train alongside twentysomethings and either hold my own or kick their arses. 
Yesterday one of the (amazingly fit) instructors in my gym told me that she and another (amazingly fit) instructor had been talking about me and saying "what a beast" I am because I train so hard, and how they hope they're as fit as me when they're my age. They're both in their late 20s so in their eyes I'm probably about age 108 but I still appreciated the compliment.
I thought it was all downhill and I was destined to feel tired, and grumpy, and invisible as a middle aged, peri-menopausal woman for the rest of my life but I was so wrong. It's not over yet. If I can do it, you all can! It won't be easy but it will be worth it.