How long should I be on the treadmill for?

I am finally able to start working out again, and I have been going to the gym at least 3 times a week. I am easing myself back into working out after not being able to because of an illness.

So, I was wondering how long I should be on the treadmill. I have been going for 25 minutes, at the target heart range on the cardio setting.

It makes the incline go up around 10-14, and I am sweating a good amount. I feel good after the workout and sleep well too.

I am thinking of bumping it up to 40 though because last time 25 was too easy.

What do you guys think?
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I am walking on the treadmill, not running. I can't run!
How long should I be on the treadmill for?
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