Swim to France. Run to Versailles.
There are better and more secretive ways besides the gym for a super-strong, super-enduring, super-fat person to lose weight.
Swimming is energy intensive since it uses many muscle groups at once, doubly so if the water is cold, since you burn calories fighting the cold.
Your character wakes up before dawn, runs to the coast and starts swimming. They swim into the deep ocean where no one else can see them. After a few days swimming around -- Here I assume super-endurance includes super-not-getting-sleepy -- they swim back to land in the middle of the night.
+10 bonus points for each shark they punch in the face on the way.
(1) If the character wants a normal life, training for days-at-a-time regularly will not mesh with work.
They only need to give up a "normal life" temporarily. The character described in the question does not need to swim to France every day to remain fit. They only have to do it long enough to shed their excess weight. So they can simply only swim on weekends, or simply not have a job for long enough to complete the project. Then afterwards they can return to a normal life.
If necessary they can go full hunter-gatherer during this time to avoid paying rent etc. For example sleeping on a deserted Pacific island and gathering food by clapping their hands to stun schools of fish and knock birds and coconuts out of the sky. Of course if they already have a family at home to support they cannot afford to disappear like this.
(2) Speed, if the character compresses the time by just running/swimming fast, we're back to square one.
That's why you run at normal human speed, to appear as a normal jogger, and only swim full speed one you get out of sight of land.
(3) Position, being spotted swimming in the middle of the ocean will require some explanations.
I think you are overestimating the visibility of a single swimming human at 3 miles (distance to the horizon). They might be visible as a "swimming object" but they will see any ships long before the ship can recognize them as a human. Then they can just swim away.