What constitutes a "hard workout" is relative: Your warmup may actually be another person's full workout.
If you join a group for the first time on a day they are doing a landmark workout, you may want to pull back and go at your own level of fitness if you are not used to the volume in the routine. Whether it is repetitions, weight, mileage, or just a long amount of time required to complete the task, a logical progression to get to the new workout level is the only way to proceed.
Throughout the fitness world, you will find avid exercisers challenging themselves with racing events on their 30th, 40th, 50th and 60th birthdays -- sometimes even older.
For instance, people have challenged themselves with a marathon on a decade birthday or perhaps 30 pullups on their 30th birthday. I had a friend who hit 50 pull-ups at 50 years old. These challenges are for those who may be getting older but are still young at heart and very well-conditioned to be able to even attempt the challenge. This year is no different.
For my 50th year, my training group and I will challenge ourselves by doing the following:
50 different calisthenics, weight, and TRX exercises (upper body, lower body, core), such as a variety of pushups, pullups, dips, crunches, plank pose (50 seconds), squats, lunges, and many more. You can make these as tough or as easy as you prefer.
However, in between each 50-rep exercise, you have to run 50m. So plan to do this one on a football field and bring dumbbells, kettlebells, TRX, bands, and other exercise aids. Most of the exercises will actually be calisthenics as in Navy SEAL Workouts (Grinder PT). Next, you get to swim 50 laps in a 25m pool to total 2,500m distance; or run 50 minutes; or ruck 50 minutes with 50 lbs.
In a separate cardio-only workout later in the week, my group will do the 50-50-50 triathlon: 50 laps/2,500m swim with fins, 50-minute run, 50-minute ruck with 50 lbs.
That is one way to celebrate your birthday with a group of tactical athletes and special ops candidates.
Over the last five years, my group has done this exact workout with 45, 46, 47 and other annual challenges, so it is not a shocker to the system. We have also done fun holiday workouts such as the Memorial Day Murph, Thanksgiving Day Feast Guilt-Free workout, and even a Super Bowl Sunday Guilt-Free workout challenge. It typically involves a little more volume than we are used to, but the large variety of exercises makes it more of a fun group challenge -- part gut check -- than an overly challenging workout that causes people to hurt themselves.
My advice is to build up to it. Take a few months practicing the events of the Murph, or multiple exercise volume workouts, or high-rep calisthenics max rep challenges.
Be smart, and you can avoid injury. Jump into something unprepared, and you could be down with a variety of overuse injuries.