Soccer works different muscles as well as working the same muscles in a different way. If you push through it, does the burn go away like your burning off lactic?
I'd say that if you're doing XC, raise your saddle high enough to give your leg a "slight" bend when it's at the end of pedal stroke. This will increase efficiency. If you're riding technical terrain...grit your teeth and bear it. Lower saddle helps for techy stuff.
Its probably like the above posters stated that your just working different muscle groups. But personally ive noticed that my diet IE what i eat the night before and the morning of a ride will have huge effect on when my legs start burning and my ability to push through it.