No one ever said getting fit over 40 was easy.

But Jill Brown, a 57-year-old certified master health fitness and nutrition coach, said it absolutely can be done — as long as you’re willing to make some adjustments to how you’re used to doing things in your 20s and 30s.

“I completely changed my routine as I approached my 40th birthday and got into the best shape of my life, then I did it again at 50,” she told The Post.

Reduce your idle time

More movement is the name of the game, especially if you’re stuck in your seat at an office job from 9 to 5.

“To get into a calorie deficit in order to lose fat, we can simply be less idle,” Brown said. “If you work at a desk setting a timer to do 20 jumping jacks or 10 sit and stands every hour will help you burn more calories over the day.”

Cleaning and other things on your to do list also count toward your goals.

“Chores around the house, taking stairs, doing your own shopping instead of using Amazon or Instacart, all add up to burning more calories,” Brown added.

Change your routine

Just because your gym routine kept you in shape for years doesn’t mean it’ll work forever.

“Our bodies adapt to stress. Exercise is a type of stress,” Brown explained. “The more you do a particular exercise or routine, the more efficient you become at it.”

While that sounds good in theory, burning more calories means you need to actually be less efficient.

“Doing workouts that are different and challenging will burn more calories than the one you are familiar with,” she went on. “However, you can fix that by simply increasing the length or the intensity of the workout (by using heavier weights for example or doing more sets and reps).”

She’s particularly a fan of circuit workouts and taking fewer breaks in between sets, which means you’ll burn more calories per session. One way to do that is to choose lighter weights but do more sets and reps, since you may need less time to rest in between.

Rest, rest and more rest

That isn’t to say rest isn’t super important, especially as we get older.

“When we’re younger, we need less recovery because everything, including our muscles, heal faster,” Brown noted. “We need to be cognizant of getting enough recovery as we age if we’re doing harder or more workouts.

“We also need more sleep as we age because we recover slower. Most of our muscle recovery happens when we sleep and rest. And lack of sleep also triggers more hunger hormones.”

Yes, you can still have abs

Brown insists you absolutely can have abs in your 40s and 50s, but it’s going to take a lot of hard work.

To get a toned tummy, “planks done properly — butt level with the shoulders if you’re on your elbows, with the butt cheeks tucked, belly pulled tight — are your best friend,” she stressed.

“I think it’s ideal to do a variety of different planks. Tip: You will actually feel elbow planks more in your abs than on straight arms.”

She also likes pilates ab exercises for strengthening and slimming your middle.

“Even though the fat burn comes mostly from a calorie deficit, we all want to make our ab and core muscles stronger. Always remember, you can’t out exercise your diet after a certain age!”