Texture/Density:
* The thick mud felt as if it were a heavy blanket draped over my feet.
* The thick mud felt as if it were molten chocolate sucking at my shoes.
* The thick mud felt as if it were a thick paste that wouldn't let me move.
* The thick mud felt as if it were a sticky, gooey taffy that clung to my legs.
* The thick mud felt as if it were a thick, viscous syrup that slowed my every step.
Movement:
* The thick mud felt as if it were a giant hand pulling me under.
* The thick mud felt as if it were quicksand slowly dragging me down.
Other Senses:
* The thick mud felt as if it were a warm, wet sponge absorbing my energy.
* The thick mud felt as if it were a heavy weight pressing down on my limbs.
You can also use similes that evoke specific feelings:
* The thick mud felt as if it were a suffocating embrace that threatened to consume me.
* The thick mud felt as if it were a hopeless burden that I could never escape.
The best simile for you will depend on the specific context and the emotions you want to evoke.