Re: How hard do you have to work to get Rhabdo?
As others have said, the likelihood of coming down with rhabdo is dependent on fitness levels and other conditions.
The military is well aware of this and that's why they push hydration so hard in the Marine Corps (I don't know about other branches). If you're continuously hydrating then you greatly decrease the chances of being sidelined with rhabdo although you may still technically have it.
The only really big issue with exertional rhabdo is if it is so serious as to cause hyperkalemia. If this is the case, the heart may cease to function properly and unless care is given ASAP death is certainly possible. The potassium levels in the blood will tend to peak within 12 hours of the event so if you didn't die by then, you probably wont.
The secondary issue is tubular necrosis leading to reduced kidney function and potentially failure. This is a longer-term problem and is where the hydration comes into play. By hyperhydrating you greatly reduce the likelihood that kidney damage will occur. This is the standard treatment in an ER setting although they also include certain types of diuretics and other medications to manage potassium levels. At the same time you must make sure you do not get freaked out that you might have rhabdo and start chugging water like crazy. You will get hyponatremia (low salt levels) which also causes problems with your heart. Standard treatment I've read consists of 1L/hr of fluids but that was under the supervision of ER staff.
If you don't die quickly (within 12 hours) from hyperkalemia and you hydrate adequately then you have little risk of having serious issues with rhabdo. It's not uncommon for experienced athletes to have high serum or urine CK levels after intense exercise (so technically having rhabdo) but they will usually remain asymptomatic. If you have extreme pain, swelling, or other issues you would consider out of the ordinary then seek medical attention immediately.