Being in shape doesn't always mean six pack abs and arms that look like cannonballs. Training for life is key and the best way to do it is to maintain your body as best as possible as injury/pain-free as possible. It's always an adrenaline to test your numbers and how much you can lift at a certain age and your endurance levels plus keeping up with a tight body those are all cool things but the truth is you won't always be able to maintain the same things in your 80's like you did in your 20's or 30's.
For those of us in our 20's and 30's are in the prime of our testosterone peaks, we can handle stress far greater than most twice our age and we feel invincible at times. I trained very hard to get where i'm at today but once I hit a certain age I do need to train differently although I'm mostly a bodyweight guy, I tend to just stick to the basics (Push-ups, Squats, Bridging & some Ab Work) while I also move around heavy hammers, qi gong exercise and Self-Resistance Training. Train to help you live longer, not beat you down and feel like a cripple.
Momentary Fitness is the type of training that is for the extremely hardcore fanatics that take exercise to levels that doesn't need to be done. Crossfit in most cases does training that lasts momentarily until someone ends up in the hospital and needs a hip replaced before their prime or have shoulder surgery and is out for 3 months. Other similar systems don't look for the long-term, just drive your body into the ground and after a few years even at age 30 some might feel like an 80 year old man; that's truly messed up and whoever pushes themselves that hard aught to get their brain scanned for insanity.
I do believe in challenges and testing yourself but not on a frequent basis and go to extremes just to prove what you can do. Most of the time people go to extremes to impress others just so they feel approval and tell everyone how fit they are. Being fit is not about impressing others or pushing yourself to keep up with someone else, its truly how you keep up with your persistence and progressing little by little and getting better at something overtime. I'm not the fittest guy in the world, no one is but I can do things at my age, my size that most can't; its not bragging (just a tad) its because I have proven to myself not someone else that I can do something and get better at it or do things because I want to and not get injured. I've had enough injuries in my time that its better to stay within a range that works and keeps me happy. I've broken both legs, cracked my head open, dented my knees, scrapped my arms, nearly cracked my ribs, taken shards of glass out of my bare hand and have been so stiff everything hurt if I moved so don't tell me about your little boo boos on your elbows or cry about having busted shoulders from your bench pressing its not going to fly.
Do things that make you feel young and vibrant, things that help you have energy well beyond your years and do things that challenge you but never need to go to the hospital for. I've read lists of people who have been in the hospital for injuries from exercising too hard. I never once in over 10 years gone to the hospital because of something from an exercise. I rarely ever have been in the ER the last few years and that is due to my judgement and how I keep my body healthy. The only time I went to the ER since my accident was for a stomach virus and was seriously dehydrated that's it.
Train with purpose but look at it from a long-term perspective. Be mindful and listen to your body.